
OK, here's at least one Christian's answer:

Jesus was a JEW, not a Christian.  In this context Matthew 5:14-19 makes
sense.  Matt 5:17 "Do not think that I [Jesus] came to abolish the Law or
the Prophets; I did not come to abolish, but to fulfill."  Jesus lived
under the Jewish law.  However, He was the culmination of the promises
of the Prophets.  He came to *fulfill* the prophecies and fully obey
God's purposes.

Verse 18 says "For truly I say to you, until heaven and earth pass
away, not the smallest letter or stroke shall pass away from the Law,
until all is accomplished."  The key to this verse IMHO is the last 
phrase.  Jesus, as the fulfillment of the law, "accomplished" what the 
Law was supposed to accomplish.  

Verse 19:  "Whoever then annuls one of the least of these commandments,
and so teaches others, shall be called least in the kingdom of heaven;
but whoever keeps and teaches them, he shall be called great in the
kingdom of heaven."  Taken in the context of Jesus teaching Jewish 
people about living lives under the law, this makes sense.

In general, it appears that Jesus is responding to some criticism he 
must have received about "doing away with the Law."  That was not 
Jesus' intent at all.  He had come to earth to live the Law as it 
should be lived and fulfill the promises made by God to his 
people all the way back to Eve [Gen 3:15-The serpent will bruise your 
heel, but *He* will bruise his head.]  Jesus appeared to be "doing 
away with the Law" because he did not honor the traditions of men as 
equal to the Law of God.  He regularly locked horns with the religious 
leaders of the day because he would not conform to *their* rules, only 
God's Law.

In the Matthew passage Jesus is defending his dedication to the Law 
and defending himself against his accusors.  Almost the entire Sermon 
on the Mount (Matt. 5-7) is dedicated to helping the Jewish people 
understand the true intent of the Law, sweeping away the clutter which 
had been introduced by the Pharasees and their traditions.

In Galatians 3:23-26, Paul describes the relationship of Jesus to the 
Law in this way:

[23] But before faith came, we were kept in custody under the law, being 
shut up to the faith which was later to be revealed.  [24] Therefore 
the Law has become our tutor to lead us to Christ, that we may be 
justified by faith.  [25]  But now that faith has come, we are no 
longer under a tutor.  [26] For you are all sons of God through faith 
in Christ Jesus.

I believe this says that after Christ was revealed, the Law had 
served it's purpose, i.e. "our tutor to lead us to Christ," and
now, "we are no longer under a tutor."  The law has been "fulfilled" 
as Christ said he would do.

God, the author of the old Law, and the Christ/Man, Jesus, are the same
personality.  Therefore, the old Law and the new Testament (the "last
will and testament" of Jesus) are based on the same moral principles. 
It makes sense that many of the principles in the old Law are
re-expressed in Christianity. 

On the other hand, now that the Law has fulfilled it's purpose and 
Christians relate to God through Christ, not the Law, it also makes 
sense that new practices and new symbolisms were established to 
represent the "mysteries" of this new relationship.  i.e. Baptism 
representing Christ's death, burial, and resurrection (Rom. 6:3-8),
The Lord's supper as a memorial to His sacrifice (I Cor. 11:26), and
Sunday as a day of worship commemorating His resurrection (Matt 28:1ff,
Acts 20:7)

OK, That's one Christian's explanation.  I don't claim to have all
these issues completely settled even in my own mind and I welcome
other Christians to offer other alternatives.

Please excuse the long posting.  Thanks for your interest if you have read 
this far...