If I remember rightly 'substantia' means the underlying reality, thus we say the Real Presence. The accidents of bread and wine retain their physical properties but in reality are no longer bread and wine but the very Body, Blood, Soul and Divinity of Our Lord, Jesus Christ, the Son of God.
The word substance has many meanings in English including a philosophical meaning akin to the Latin, 'that which stands beneath'. Most Catholics will not be aware of this and need not understand Aristotelian metaphysics. Most Catholics can not be expected to be philosophers and theologians and therefore acceptance of what the Church teaches is essential.
In this matter the Church says to its members that the bread and wine cease to exist at the Consecration. From then on they are the actual Body and Blood of Christ regardless of whether one believes it or not: it is, in fact a miracle effected by the Words of Institution.  If you believe it you are a Catholic, if you don't, you are not.