Sorry it took a while to respond, I wanted to be thorough in compiling some scripture.
I may have to put each response in it's own post due to space.
First, compare Mat. 5:17-19 with Ephe. 2:15. While Jesus said that he had not come to abolish the Law, Paul said that the Law was abolished on the cross. Paul gives us the impression that Jesus did not know what he was talking about.
Matthew 5
17 “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly, I say to you,
until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.
The following are questions you need to ask yourself, and be able to answer, if you wish to understand it's meaning:
-If He were talking about obedience to the Mosaic law then why even mention the Prophets? There's more to what He is saying here than an observance of
the Mosaic covenant's conditions.
-How does Jesus define the Law and the Prophets?
-He says He has not come to abolish them, but to complete them (Greek: Plerosai)
How exactly is He going to complete the Law and the Prophets?
-He implies that the Law actually can pass away, and gives two conditions for this. The first is that heaven and earth pass away.
The second, is that the law stands until it has been accomplished.
What must be accomplished?
Understanding what he means here requires a larger understanding of who He was and what He did based on the scripture:
Jesus is Messiah. He is the one whom the Law and Prophets refer to.
His arrival and actions are the completion of what they spoke of.
He is also the fulfillment of the promise God gave through Moses and the Prophets.
He is also the perfection of the law embodied in mankind.
John 5
39 You search the Scriptures because you think that in them you have eternal life; andit is they that bear witness about me, 40 yet you refuse to come
to me that you may have life.
John 1
45 Philip found Nathanael and said to him, “We have found him of whom Moses in the Law and also the prophets wrote, Jesus of Nazareth, the son of
Joseph.”
When he said he had not come to abolish the Law and the Prophets, but to complete them, let's look at how Jesus defines the Law and the Prophets:
matthew 7:12
12 “So whatever you wish that others would do to you, do also to them, for this is the Law and the Prophets.
Matthew 22
36 “Teacher, which is the greatest commandment in the Law?”
37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’[c] 38 This is the first and greatest
commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’[d] 40 All the Law and the Prophets hang on these two commandments.”
Luke 10:
26 “What is written in the Law?” he replied. “How do you read it?”
27 He answered, “‘Love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind’[c]; and, ‘Love
your neighbor as yourself.’[d]”
Jesus did not come to abolish the Golden Rule, love, which underpins everything that God instructs people through Moses and the Prophets. He came to
bring a completion to it, a perfection to it, to render it full in existance (More on this further down when we examine the greek meaning of the word behind "fulfill")
He then commands us to follow this commandment:
John 13:
34 A new commandment I give to you, that you love one another: just as I have loved you, you also are to love one another. 35 By this all people will
know that you are my disciples, if you have love for one another.”
matthew 5
48 You therefore must be perfect, as your heavenly Father is perfect.
Here He is telling us, after an exposition on what God's law really means (The spirit behind the written law, how the mosaic commands only reflect
guidelines designed to point us towards righteousness) that we must display perfect love in the same way that the Father does.
To understand what he means by fulfill, let's examine some greek words:
Matthew 8:17
17 that it might be fulfilled which was spoken by Isaiah the prophet, saying:
“He Himself took our infirmities
And bore our sicknesses.”
The greek word for fulfilled here is the same one used in matthew 5, and the same one used throughout scriptures like this - Pleroo.
Pleroo carries the conotation of the completion of something, to render full, to fill up to fullness.
This word is used many more times in the gospel with reference to Jesus "fulfilling what was spoken by the prophets".
luke 22:37
37 For I say to you that this which is written must still be accomplished in Me: ‘And He was numbered with the transgressors.’ For the things concerning
Me have an end.”
The word used for "accomplished" is telesthenai. It implies to accomplish, execute, perform, fulfil.
The word used for "end" is telos, which implies completion, perfection, connsumation, an end, a closure brought to something with results.
This very clearly outlines for us the concept Jesus is communicating to us: That He is the fullfillment, the completition, the accomplishment of what
was written in the Law and the Prophets.
He also tells us that the things "concerning Him" have "an end", a closure, a completion.
Jesus tells us that Moses and the Prophets were witnessing to Him, and that in Him these things are completed and brought to a conclusion.
Ephesians 2:
14 For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility 15 by abolishing the law of
commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, 16 and might reconcile us both
to God in one body through the cross, thereby killing the hostility.
Paul is writing to gentiles here.
The subject here is putting to rest the hosility between Jew and gentile, the wall or ordinances and commands according to the law which divides them.
Under Messiah's overriding commandment of love, being fulfilled through the gift of the Holy Spirit in us, there is no need for the ritual observances
of the Mosaic ordinances when we have God's law written on our hearts as part of the Messianic covenant. For, as Jesus and Paul both tell us, if we
follow the commandment to love then everything else falls into line with God's law. The only problem is that people lack the capacity to Love the way
God does on their own. Jesus and Paul both tell us that only by being born again with the Holy Spirit dwelling in us, and following it's leading, can we
actually follow God's law. (All the scriptures supporting this would be beyond the scope of this post though)
"Law" or "Torah" is a word denoting received instructions from God. Any instruction from God is always right and becomes law to those who it is given
to.
The Mosaic covenant was a type of law for a specific group as part of a specific covenant, but they could never keep it. They could never seem to
inherit it's blessings, only it's curses, because of their inability to follow God's instructions.
God's Spirit, through Messiah, is what empowers us to be able to actually keep any instruction He gives us by giving us a new heart.