1. No Scriptural verse exists saying that the Theotokos sinned.
-It is a Protestant tradition generally of later vintage
-Imposes tradition over the Scriptures, usually due to extra-biblical anthropological assumptions imposed upon the Scriptures (i.e temptations are sins as opposed to Heb 4:15, that having the “flesh” of Rom 7 by default makes one guilty of sins)
-Orthodox define these assumptions only very generally as sins (i.e. infants are baptized for the “Remission of sins“, but we view this as “sinfulness”)
-In plain English, Protestants will argue things that are not sins (such as avoiding an evil thought) are literal sins–this is incorrect
-The Scriptural definition of sin: “Whoever commits sin also commits lawlessness, and sin is lawlessness.” (1 John 3:4) Also, “He who does righteousness is righteous” (1 John 3:7), but “all unrighteousness is sin.” (1 John 5:17)

-Sin is something literally done and committed. Contrarily, righteousness is something done and committed. Period.
-Orthodox believe the Theotokos had original sin, so this is why we confess only Christ alone is without sin, but it is incumbent upon the Protestant to prove she committed an act of sin in order to disprove her Sinlessness.
-Hello Bible, where do you say the Theotokos sinned?  I thought Protestants were sola scriptural!
-Scripture only identifies her as a virgin, same with the earliest creeds–Protestants cannot assume anything contrary to this and pretend to abide to the regulative principle
-We cannot accuse individuals of things we do not know they did, this is sinful
2. Scriptures obliquely teach the Theotokos is sinless
The Theotokos is the Ark of the Covenant
This is relevant because the tabernacle was constructed by what was considered to be, according to McClintock and Strong’s encyclopedia, “imperishable wood.” It was likewise overlaid with gold, an “imperishable metal.” Now neither of these are strictly true, both can tarnish and deteriorate over time, but the overall point is clear.
1. The parallel between the Ark and Mary between 2 Sam 6:9 and Luke 1:43.

David was afraid of the Lord that day; and he said, “How can the ark of the Lord come to me?” (2 Sam 6:9)

“But why is this granted to me, that the mother of my Lordcome to me?” (Luke 1:43) [[Same Greek word for to/toward is used in lxx]]
The preceding parallel implies that there is a connection between the ark of the covenant and Mary. They are both footstools of God that He uses to enter the world.
2. The Tabernacle (Ex 40:29 LXX)/Mary (Luke 1:35) was “overshadowed” by God Himself. 
And Moses was not able to enter into the tabernacle of testimony, because the cloud overshadowed (ἐπεσκίαζεν) it, and the tabernacle was filled with the glory of the Lord. (Ex 40:29 LXX/ Ex 40:35 MT) 
And the angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Highest will overshadow (ἐπισκιάσει) you; therefore, also, that Holy One who is to be born will be called the Son of God.” (Luke 1:35)
3. The striking of Uzza in 1 Chron 13:10 contrasted with the proper way the Ark was transported in Num 4:5.

And the anger of the LORD was kindled against Uzza, and he smote him, because he put his hand to the ark: and there he died before God. (1 Chron 13:10; cf 2 Sam 6:6-8)

When the camp prepares to journey, Aaron and his sons shall come, and they shall take down the covering veil and cover the ark of the Testimony with it. (Num 4:5)

The preceding is important (especially in light of Num 4:15) that the ark is not to be touched lest it be defiled. This is the whole reason it was transported covered with curtains. This is how undefiled the tabernacle, and thereby Mary, is. She cannot be a defiled sinner and God’s footstool.
And so, the Protestant is forced to argue that Mary became defiled after Christ was born, as if the coming of Christ leaves us worse off than our former state. Perish this horrible thought!
Ezek 44 speaks of the Theotokos’ perpetual virginity:
the Lord said to me, “This gate shall be shut; it shall not be opened, and no man shall enter by it, because the Lord God of Israel has entered by it; therefore it shall be shut. As for the prince, because he is the prince, he may sit in it to eat bread before the Lord; he shall enter by way of the vestibule [lit. “hall” or “porch”, “chamber” in LXX] of the gateway, and go out the same way.” (Ezek 44:2-3)

The prince enters and leaves from a gateway which has a gate that cannot be opened. 
There are others, like Song 4:12, but the preceding is the clearest.

Such a doctrine implies Mary’s continued sinlessness. There is no Biblical indication she as an individual sinned.

  3. Patristic sources teach that the Theotokos was sinless from a variety of angles
Explicit comments on sinlessness
Augustine in a passage about why the Pelagians are wrong about there being sinless individuals out there:We must except the holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honour to the Lord…Well, then, if, with this exception of the Virgin, we could only assemble together all the forementioned holy men and women, and ask them whether they lived without sin while they were in this life, what can we suppose would be their answer? (On Nature and Grace, par 42) 
Ambrose:Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sarah but from Mary, a virgin not only undefiled, but a virgin whom grace had made inviolate, free of [lit. “integra” meaning having integrity from] every stain of sin. Ambrose,Sermon 22:30, Commentary on Ps 118 [[PL 15, 1521, Ut incorrupta sit virgo, sed virgo per gratiam ab omni integra [lit. intact] labe peccati.]]
Six Books Apocryphon:

The narrative also implies her sinlessness.  She is described “holy and elect of God from (the time) when she was in her mother’s womb.” (Wright 1865, 130) Additionally, when the Jewish authorities cautioned her not to pray at Christ’s tomb, admonishing her to “[r]emember the sins thou has committed before God,” (Wright 1865, 134-135) the narrative continues with saying “she did not agree” with this statement. It is left unsaid if it was the assertion that she sinned, that she leave Golgotha, or both that she objected to. 
Doctrine of preservation from sin until at least the incarnation:

Implicit in Protoevangelium of James: “[Anna to Mary:] ‘You shall not walk on this earth until I bring you into the temple of the Lord.’ And she made a sanctuary in her bed-chamber, and allowed nothing common or unclean to pass through her.” (par 6)
-she begins walking on the third step of the altar (par 7)
-par 6 at a great feast, Mary only drinks breast milk
“she received food from an angel” (par 8)

Hippolytus Fragment From the Commentary by the Holy Bishop and Martyr Hippolytus, on The Lord is My Shepherd:
And, moreover, the ark made of imperishable wood was the Saviour Himself. For by this was signified the imperishable and incorruptible tabernacle of (the Lord) Himself, which gendered no corruption of sin…[T]he Lord was without sin, made of imperishable wood, as regards His humanity; that is, of the virgin and the Holy Ghost inwardly, and outwardly of the word of God, like an ark overlaid with purest gold.
And so, being made of Mary makes Him made of imperishable, sinless flesh (i.e. wood). His divine nature, is typologically the layer of gold on the ark.
More general statements of sinlessness, which lack English translations:
Ephraem,Nisibene Hymns,27:8
“Thou alone and thy Mother are in all things fair, there is no flaw in thee and no stain in thy Mother.”

[Sources of authenticity I cannot verify that assert her sinlessness:
Origen (Hom. i in diversa–Rufinus?)
“This Virgin Mother of the Only-begotten of God is called Mary, worthy of God, immaculate of the immaculate, one of the one.”
Catholic Encyclopedia: Origen calls her worthy of God, immaculate of the immaculate, most complete sanctity, perfect justice, neither deceived by the persuasion of the serpent, nor infected with his poisonous breathings (“Hom. i in diversa”)
Gregory Thaumaturgus, Concerning the Mother of God, par 27
“For she alone was spotless in soul and body.”

Theodotus of Ancrya, Homily VI:11
“A virgin, innocent, spotless, free of all defect, untouched, unsullied, holy in soul and body, like a lily sprouting among thorns.”]

Maintained her virginity in her childbirth (all 1st or 2nd century)
-Ascension of Isaiah 11:12-14 (“The story of the child was noised abroad in Bethlehm…Many said: ‘She has not born a child, nor has a midwife gone up (to her), nor have we heard the cries of (labour) pains.'”)

-ignatius (smyrnaeans chap 1) and irenaeus (ah book 3 chap 4 par 2) quoting a creed that says Christ was “born of a/the virgin”
-Hegesippus (n.d.) in a fragment wrote, “There still survived of the kindred of the Lord grandsons of Jude, who according to the flesh was called His brother.”  The obvious implication is that Jude was not Jesus Christ’s actual brother, which is an oblique reference to the Theotokos’ perpetual virginity.
-Protoevangelicum of James, which teaches that the Theotokos maintained uncorrupted Virginity as verfied by a gynecological examination (Par 19-20)
-Ode of Solomon 19: “she labored, but not in pain.”  Also calls her “THE virgin.”

Mary’s Perpetual Virginity

St Siricus’ condemnation of the Jovinianists in ambrose corpus calls these heretics, which impugned chastity and the doctrine of the Theotokos’ perpetual virginity, “the promoters of the new heresy and blasphemy” by “the unanimous sentence of us all, as well of the presbyters and deacons as of the other clergy.” In other words, this was a new idea that everyone who was a clergyman rejected.

[[6. Having held am assembly of my clergy…Therefore, following the Apostolic precept, we, seeing that they were preaching another Gospel than that which we received, have excommunicated them. Know therefore that it was the unanimous sentence of us all, as well of the presbyters and deacons as |282 of the other clergy, that Jovinian, Auxentius, Genialis, Germinator, Felix, Prontinus 9, Martianus, Januarius, and Ingeniosus, who were discovered to be the promoters of the new heresy and blasphemy, should be condemned by the Divine sentence and our judgment, and remain in perpetual exclusion from the Church.]]

Ambrose Letter 42 (the council of milan’s statement against the Jovinianists) concurred all clergy taught the perpetual virginity of Mary:

4. [F]rom their perverse ways they are induced to say ‘She was a virgin when she conceived, but not a virgin when she brought forth.’ Could she then conceive as a virgin, and yet not be able to bring forth as a virgin, when conception always precedes, and birth follows?

5. But if they will not believe the doctrines of the Clergy, let them believe the oracles of Christ, let them believe the admonitions of Angels who say, For with God nothing shall be impossibleFor thus it is written, Behold a Virgin shall conceive and bear a Son; declaring not only that she should conceive as a virgin, but also that she bring forth as a virgin.

They then continue and give the exact same exegesis of Ezekiel I did:

6.  But what is that gate of the sanctuary, that outward gate which looketh towards the East, which remains shut, and no man, it is said, shall enter in by it but the Lord, the God of Israel. Is not Mary this gate, by whom the Saviour entered into the world? This is the gate of righteousness, as He Himself said, Suffer us to fulfil all righteousness. Blessed Mary is the gate, whereof it is written that the Lord hath entered in by it, therefore it shall be shut after birth; for as a virgin she both conceived and brought forth.

[[Par 14 has the subscription of about 10 bishops.]]
Comments on Mary’s “Sin” At the Cross
The fathers have a consensus about Mary’s “sin”: the closest the Theotokos came to “sinning” was almost-doubting Christ at the cross due to her grief. Yet, this would not even be a committing of an act and so this would not meet the Scriptural bar for sin.
Origen:What ought we to think? That while the apostles were scandalized, the Mother of the Lord was immune to scandal? If she had not experienced scandal during the Lord’s Passion, Jesus did not die for her sins. But if “all have sinned and fall short of the glory of God” and if all “are justified and saved by his grace” (Rom 3:23), then Mary, too, was scandalized in that moment. This is what Simeon is prophesying about: … Your soul will be pierced by the sword of unbelief and will be wounded by the sword point of doubt. (Homily on Luke 17, 6-7; PG 13, 1845; SC 87, 236-58)

Brian Reynolds: “His intent here is not to assail the holiness of the Virgin, but to ensure the princpal [sic] of Christ’s universal redemptionis upheld…even her momentary loss of faith, was not to denigrate her in any way, but rather to set her up as an exemplar of Christian perseverance in the face of adversity and temptation.” (Gateway to Heaven, Vol 1, 249)
Basil:About the words of Simeon to Mary, there is no obscurity or variety of interpretation…Simeon therefore prophesies about Mary herself, that when standing by the cross, and beholding what is being done, and hearing the voices, after the witness of Gabriel, after her secret knowledge of the divine conception, after the great exhibition of miracles, she shall feel about her soul a mighty tempest…Even you yourself, who hast been taught from on high the things concerning the Lord, shall be reached by some doubt. This is the sword. That the thoughts of many hearts may be revealed. He indicates that after the offense at the Cross of Christ a certain swift healing shall come from the Lord to the disciples and to Mary herself, confirming their heart in faith in Him. (Letter 260, Par 6, 9)

Ambrosiaster/Hilary of Poitiers:As for what Simeon adds: “And the sword will pierce your soul, so that the thoughts hidden in the depths of the hearts of many will be revealed,” (Luke 2: 35) indicates that Mary, in whose bosom the mystery of the incarnation has been wrought, and there has been some doubt at the death of Our Lord, but doubts that the resurrection’s brilliancy and the Savior’s power soon changed into a firm and unshakable faith. At the death of the Savior, all under an impression of dread, let doubt enter their souls. However, they did not persevere in doubt. (Questions and Answers on the Gospel of Luke, Question 73)

//only if time…
(Saint Cyril of Alexandria, Commentary on John, Book 12) For, doubtless, some such train of thought as this passed through her mind: ‘…He said, indeed, that He was the true Son of Almighty God, but it may be that He was deceived; He may have erred…The woman, as is likely, not exactly understanding the mystery, wandered astray into some such train of thought…And no marvel if a woman fell into such an error…By a sword, he [Symeon] meant the keen pang of suffering, which would divide the mind of the woman into strange thoughts, for temptations prove the hearts of those that are tempted.

(Saint Hilary of Poitiers, Homily on Psalm 118:20) Seeing we must render an account for every idle word do we desire the day of judgment in which that unwearied fire is to be passed through in which those grievous punishments are to be undergone for the expiating of a soul from sin, a sword shall pass through the soul of the blessed Virgin Mary that the thoughts of many hearts may be revealed. If that Virgin who bore God is able [capaz illa Dei Virgo ventura est],to come into the severity of the judgment will any one dare desire to be judged by God?
4. Western anthropology is defective, which is why they do not understand Christology or Mariology

In summary, 1. the Protestants have no Scriptural proof that Mary sinned. 2. The Scriptures clearly teach she was sinless. 3. We have beginning in the first century Christian sources which teach doctrines auxiliary to her sinlessness and in the fourth century both Greek and Latin sources which explicitly assert her sinlessness–I count a total of 14 verifiable sources which by ancient history standards is a tidal wave. As I showed, the Theotokos’ grief at the cross was not sinful. 4 Confusion over the preceding matter is due to the West not understanding the anthropology to the 6th ecumenical council, as no one who understands this would confuse doubt as an actual committing of sin.

___(Chrysostom, Homily 44 on Matthew, chap 2:) chap 2: “neither did He declare and pronounce judgment against them; but He yet left in it their own power to choose, speaking with the gentleness that becomes Him.”
(Chrysostom, Homily 44 on Matthew, chap 1, 3:) (…”these are my mother and brothers”) For in fact that which she had to do, was of superfluous vanity; in that she wanted to show the people that she has power and authority over her Son…Whence it is clear, that nothing but vainglory led them to do this…For this cause He quite repels them, being minded to heal their infirmity…out of care for her, and for His brethren. I mean, because their regard for Him was as towards a mere man, and they were vainglorious, He casts out the disease, not insulting, but correcting them…with what purpose He reproved; that it was not with intent to drive them to perplexity, but to deliver them from the most tyrannical passion and to lead them on little by little to the right idea concerning Himself, and to convince her that He was not her Son only, but also her Lord…3. on this occasion too, He both healed the disease of vainglory, and rendered the due honor to His mother, even though her request was unseasonable.

[Note: In fuller context, it appears that the brothers were actually vainglorious and Mary could have fallen prey to it…perhaps Christ was trying to pre-empt Mary from willing wrongly from a natural impulse of a mother who could have excessively prided herself for having a child of “good report,” consistent with the annunciation and “the sword” where God intervened to prevent an unnatural passion from occurring. For example: Whence it is clear, that nothing but vainglory led them to do this; which John too declares, by saying, Neither did His brethren believe in Him; John 7:5 and some sayings too of theirs he reports, full of great folly; telling us that they were for dragging Him to Jerusalem, for no other purpose, but that they themselves might reap glory from His miraclesFor if you do these things, it is said, show Yourself to the world. For there is no man that does anything in secret, and seeks himself to be manifest; when also He Himself rebuked them, attributing it to their carnal minds…the unbecoming conduct of His brethren, and the boldness wherewith they had been bold and who was the person reproving it, no mere man, but the only-begotten Son of God; and with what purpose He reproved (Chap 1)]

[[[chap 2: neither did He declare and pronounce judgment against them; but He yet left in it their own power to choose, speaking with the gentleness that becomes Him.” so, mary did not explicitly sin, there was no judgement]]]

Ex 4:22 Israel is my firstborn son, law states people must still sacrifice for their firstborn son, even if he is an only child (pidyon haben, lit “opener of the womb”), see ex 13:12–luke 2:22-24 requires usage of term firstborn

Jovinian and Helvidius were the liberal heretics of their day. The former said that because st Paul said “says I not the Lord” that the apostle was essentially incorrect. The latter would accuse passages of scriptures that no one disputes today that were inconvenient to his arguments to be “forgeries.” Both were not bishops, both were former monks I believe. They didn’t speak for a historical belief system, they spoke for themselves.