MATTHEW v 17-20
The opening words suggest the motive from which these verses take their start. "Think not" (repeated somewhat similarly in MATT. x 34) was not likely to have been said unless there was some real probability that without the warning the disciples might think as they are here bidden not to think. It was easy to misunderstand the true purpose of the new prophet who had appeared going about Galilee, teaching in the synagogues, proclaiming the Gospel of the kingdom, and healing all manner of sickness and infirmity. Signs of His coming antagonism to Scribes and Pharisees, the jealous guardians of the Law, had possibly already appeared. At all events the tone and drift of His teaching was manifestly unlike theirs. Thus it was not unnatural to assume hastily that it was a purpose of His mission simply to beak down restraints, to lift from men's shoulders the duties which they felt as burdens. The Law was full of commandments which claimed to be obeyed. The Prophets were full of rebukes of transgressors, and warnings of coming doom. Might not the mild new Rabbi be welcomed as one come to break down the Law and the Prophets, and so lead the way to easier and less exacting ways of life?
This is the delusion which our Lord set Himself to crush. The Gospel of the kingdom was not a Gospel of indulgence. "Think not that I came to destroy the Law or the Prophets (to pull them down, undo them:- both these shades of meaning meet in katalu,oai): I came not to destroy but to fulfill." These last two verbs are doubtless absolute: not as regards Law and Prophets only, but as regards all things, not destruction but fulfillment was His characteristic work. But this was especially true for the Law and the Prophets. About the word "fulfill" (plhrwoai) there is a certain ambiguity. But we may safely neglect the meaning which perhaps comes first to mind, that of personal obedience or performance, as we speak of the fulfillment of an injunction. The true meaning answers much more exactly to that destroying or undoing to which it is here formally opposed. It is to bring to fullness or completion, involving therefore a progress: it is not to keep a thing as it was.
from pages 14-16, Judaistic Christianity