Matthew 5:17,18 - The Fulfilling of the Law

Matthew 5:17 - Don't think that I came to destroy the law or the prophets. I didn't come to destroy, but to fulfill.

Matthew 5:18 - For most assuredly, I tell you, until heaven and earth pass away, not even one smallest letter or one tiny pen stroke shall in any way pass away from the law, until all things are accomplished.

Something we should note is that until the apostles were enlightened by the holy spirit there was no general understanding of the basis of salvation through the promised Messiah, although some typical arrangements had been provided to typify the sacrifice Jesus was to give. God, however, who calls things that are not as though they were (Romans 4:17), could and did bring a reckoning of salvation before the sacrifice of Jesus was made, since God had foreknowledge that the sacrifice was to be given, and thus due to his foreknowledge, that which was not yet was reckoned as being as though it had already happened, just as he counts people as alive in view of the coming resurrection in the last day. -- Luke 20:37,38.

The everlasting law covenant with Israel was still upon the Jew in general when Jesus gave his sermon on the mount, and it is still in effect upon those still under the law, although it can not give life to anyone due to the sinful flesh. (Romans 8:3; Galatians 3:12) "For as many as are of the works of the law are under a curse." (Galatians 3:10) The law -- as a covenant -- is never destroyed, but it does "pass away," 'vanish,' (Hebrews 8:11), once all of its subjects have died, either actually or reckonedly. "The law has dominion over a man for as long as he lives." (Romans 7:1) "Now we have been discharged from the law, having died [reckonedly -- Romans 6:11] to that in which we were held." (Romans 7:6) The Law Covenant was designed only for man in his sinful state; it was not designed for the righteous, the justified, the child of God, who has no sin. (1 Timothy 1:9) Thus, as each of the natural sons of Israel dies, whether physically, or by being reckoned dead through faith in Jesus, the law has passed away for that natural son of Israel. Once all the Jews have either died physically or reckonedly, the Law will have vanished. -- Hebrews 8:11.

This is because the law covenant was not given to all mankind, but only to the nation of Israel. (Exodus 19:5; Deuteronomy 7:6; Amos 3:2) Gentiles who are grafted into the ecclesia of Christ are not grafted into the law covenant, as some have assumed, for if they were, it would only serve to be a curse for them. (Galatians 3:10) It would further mean that they had not been justified (declared righteous), since it would still account them as sinners. (1 Timothy 1:9) All Christians are, however, under the Law of love, which is the spirit of the Law, and thus all the commandments in the Law are to be obeyed in a spiritual way, in a magnified manner, without becoming bound to the Law itself. Thus, the law of love is counted as fulfilling the Law. -- Romans 13:8,10.

Therefore, Jesus, as the seed of promise (Genesis 3:15; Isaiah 9:6,7), the promised son of the man, David (Acts 13:23), had already begun to use his authority as the promised son of David to magnify the Law. Jesus' purpose was not to destroy the law, but to fulfill and establish the justness of the Law. The covenant with Abraham and his seed had already begun long before the Law Covenant. (Galatians 3:16,17) In many cases, it is as the "seed of Abraham" that Jesus acts. Jesus, of course, having received a body especially prepared by his God and Father (Hebrews 10:5; John 6:33,51), did not have the condemnation of Adam, as the rest of mankind. (Romans 5:12-19; 1 Corinthians 15:21,22) And yet, he, although not a sinner, was born under the law. (Galatian 4:4) How could he, a righteous man, be under the law, when the law was made for sinners? The prophet wrote that "Jehovah has laid on him the iniquity of us all." (Isaiah 53:6) This, of course, was in a reckoned manner, not because Jesus' flesh was actually sinful. Jesus came into the world of mankind in the "likeness" [not actually, but similar to that] of sinful flesh (Romans 8:3) only because Jehovah had laid upon him the iniquity of the whole world. He suffered and died as though he were a sinner, although in reality, he wasn't. And yet, through his obedience, Jesus condemned sin the flesh. -- Romans 8:3.
See:

How God's Son Condemned Sin in the Flesh

The fulfillment of the law, however, requires not only the keeping of the commandments that are peculiar to the children of Israel, but rather in the fulfillment of the typical arrangements provided in the law through the realities that are pictured by those types. A type is not the reality, but the antitype is the reality. (Hebrews 10:1) Many seem to get this backward, as many seem to think of the type as the reality, and the spiritual fulfillment as not actually being the real thing. Nevertheless, many of these types involve the Ecclesia of Christ, and thus, the fulfillment of the law extends beyond Jesus himself, although Jesus is still the one bringing about that fulfillment. (Colossians 2:17; 8:5; 10:1) Since some of these typical arrangements are not fulfilled until the age to come, then the present heaven and earth must pass away before the law is totally fulfilled. Then, through the New Covenant in the age to come, righteousness will begin to prevail, not just among the children of Israel, but among all nations.  -- Genesis 18:18; 22:18; 26:4; Psalm 72: 11; Isaiah 2:2-4; 42:6; Revelation 21:1-5; 22:2.

Additionally, Jesus had to be born under the law, and to suffer as though he were disobedient to that law, else he could not have purchased, redeemed or delivered any from the curse of the law. -- Galatians 3:13.

Romans 4:13 - For not through the law was the promise to Abraham or to his seed that he should be heir of the world, but through the righteousness of faith.
Romans 4:14 For if those who are of the law are heirs, faith is made void, and the promise is made of no effect.

Jesus' inheritance was not of the law covenant (Galatians 3:18; Romans 4:13), although Jesus, being born under the Law (Galatians 4:4), was obligated to keep the Law covenant (Romans 3:19; James 2:10; Romans 7:1), which he did perfectly, else he would have been cursed under the Law just as the Jews in general. (Galatians 3:10) If he had failed in one small matter (James 2:10), he would not have received the inheritance, since he would be condemned just as everyone else under the law. While his keeping the law perfectly proved him to be the promised one, his inheritance as the promised one was due to his being the seed of Abraham, the seed of David, not due to the law covenant itself. Likewise, the believers' inheritance is because they are made the seed of Abraham with Jesus, and it is not attributed to the law covenant, nor is directly attributable to the new covenant, but the scriptures do appear to say that one has first become a son by appropriating the blood of the new covenant and its power from the age to come before he can become a member of the seed of Abraham. -- Matthew 26:28; Mark 14:24; Luke 22:20; 1 Corinthians 11:25; Hebrews 6:5; 10:29; 12:22-24.

We should realize that the new covenant does not have its actual "beginning" until the age to come, when God, by means of that covenant releases the dead from the tomb through the application of the blood of Christ. There could be no resurrection of either the just or the unjust without an application of the blood of the new covenant that releases the dead so that they may be raised in the last day, whether to life or judgment. Nevertheless, the believers in this age receive the application of the new covenant through the holy spirit as a token, a down payment, an earnest, of the reality to follow. (2 Corinthians 1:21,22; 5:4,5; Ephesians 1:13,14) Being transferred thereby into the coming kingdom, they become ambassadors of that kingdom and ministers of the new covenant.

The new covenant, however, is not just for the purpose of releasing the dead from the death condition resulting from Adam, but it is also for the purpose of reconciliation (Romans 5:11; 2 Corinthians 5:19), returning mankind not only to what Adam had before Adam sinned, but also returning mankind to the possibility of perfecting love and faith in the Creator, as Adam could have done. This, of course, does not mean that any released from the condemnation in Adam becomes lawless. Through Christ's obedience to God, including his obedience to the law covenant, as a man, and due to his proving himself unswervingly incorruptible as a man, he brought life and incorruption to light. (1 Corinthians 15:54; 2 Timothy 1:10) A sinless man (Jesus) had not only kept God's law, but had also proven himself to be incorruptible in obedience to his God and Father.

Likewise, the prophets are not yet fulfilled, and will not be fulfilled until all creation has been fully brought back into accord with God through Jesus. This does not happen until after the thousand years of the reign of Jesus and the church so it does not happen until after the present heavens and earth will have passed away.