A confusion about the fullfilment of the law

In Romans 5:12-21 this is explained:

1. Through the disobedience of one man, sin came into existence.
2. Consequence was that death "ruled as king", meaning it was inevitable for all people, even for those who had not been disobedient like Adam.
3. Death was not a consequence of the Law: it was there before the Law.
4. The Law defined "sin", so that there were legally established consequences; and
5. Life can be gained through undeserved kindness by means of Jesus Christ, because of one act of justification.

One act of justification made up the balance versus one act of disobedience. How so? In Romans 3:23 we read the same statement, that "all have sinned and fall short of the glory of God", but the next verse calls that one act of justification a "release by the ransom paid by Christ Jesus". In 1 John 4:10 it is called a "propitiatory sacrifice". Why is the sacrifice of equal value, and to what? Adam lost his perfect life through his disobedience. Jesus was also a perfect man, without sin, and gave up his life even though he was not deserving of death (1 Peter 2:22,24). Because Jesus was, as a Jew, under the authority of the Law, this could also be legally established. So a life for a life.

That is why the Bible says that while people die from inherited sin because of their forefather Adam, they also have an opportunity to gain life through faith in Jesus' ransom sacrifice, so that in effect, Jesus becomes their (spiritual) father. That is why Jesus does not have to be the Almighty God to be called Eternal Father (Isaiah 9:6), because the effect of the ransom lasts forever (Hebrews 7:27). To illustrate, Abraham is also called "the father of all those having faith" (Romans 4:11,12), which goes beyond his physical offspring. Note that Paul also calls Abraham "the father of us all", which most likely would have included Roman Gentiles who did not have Abraham as their ancestor (Romans 4:16).

One could illustrate the above with a building contract. Does it last forever? No, it exists and is legally enforcable, until the building has been fully constructed according to all the terms of the contract. Once that has been accomplished, the contract is history. Not by ripping it apart, but by complying with all the terms. That is what Jesus did in connection with the Law of Moses from the beginning of his life, even when he had no control over it (Luke 2:21-24). Paul also notes in Romans 13:8,10 that love is the fulfillment of the law. Connect that with Jesus' words in John 15:13: "No one has love greater than this, that someone should surrender his life in behalf of his friends", and yet, he himself died even for his enemies (Romans 5:6,10)!

So in summary, Jesus was saying "There is no chance in heaven or on earth that even the tiniest part of the Law would change or be removed, until it has all been done." Once he had died, the Law had been fulfilled because of the life that Jesus lived. At that point, there was also every opportunity to make a "new covenant" that would replace the old covenant that included the Law of Moses. Jesus took that opportunity and made a new covenant, meaning that then the Law of Moses was abolished (Luke 22:20,29). Does the new covenant come with a law? Yes, it is known as the 'Law of the Christ' (John 13:34; Galatians 6:2).

In case Romans 5:12-21 is not entirely clear, or difficult to understand, there is a video that explains it very well, I think. It's entitled "Why did Jesus die?" https://www.jw.org/en/library/videos/#en/mediaitems/VODBibleTeachings/docid-502016853_1_VIDEO

That should answer your question.

As @curiousdanni pointed out in comments, the text here does not put forth the eternality of the Law and Prophets but rather highlights two conditions in which they may or will pass away: If the current created order passes away or if everything in them is fulfilled. It is possible that these two are to be understood as concurrent but that is not necessary.

Jesus said that he came for the purpose of fulfilling the Law and the Prophets, some of which has been accomplished and some of which is still yet to be. Paul makes it clear that the Law's purpose as a means for obtaining righteousness has been brought to an end. This is not a terminal end but an objective end; Jesus is the righteousness that the Law was intended to produce. "For Christ is the end of the law for righteousness to everyone who believes." - Romans 10-4 Also, the epistle to the Hebrews is filled with elements of the tabernacle and sacrificial system that typified and were fulfilled in Christ (Hebrews chapter 9). In 1 Corinthians 10:4 Christ is pointed out as the spiritual rock the Israelites drank from in the desert and there are many more such examples. The Messiah was also prophesied to need to come and to suffer and to enter into glory (Isaiah 53 and Luke 24:26 among many others). So, there are a great many aspects of the Law and Prophets which have already been fulfilled in the birth, life, death, burial, resurrection, and ascension of Jesus into heaven as well as the giving of the Holy Spirit (Joel 2:28) and the inauguration of the Church (Hosea 2:23).

But the Law and Prophets are not yet passed away. "Do we then overthrow the law by this faith? By no means! On the contrary, we uphold the law." - Romans 3:31 We do not set the Law aside because "the law is holy, and the commandment is holy and righteous and good." - Romans 7:12 What has ended regarding the Law is that it is no longer the means by which righteousness may be obtained. Christ has met every requirement of the Law for us in that regard; he has kept the Law in a positive sense, not only in letter but in spirit and he has borne the curse of the Law for us as well so that, through faith, we are now able to fulfill the righteousness that the Law required: "For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit." - Romans 8:3-4 This is why Paul speaks so harshly against those who claim that he taught a flaunting of the Law (Romans 3:8)

And there are many prophesies yet to be fulfilled regarding the second coming of Christ (Zechariah 12:10), the resurrection (Job 19:26), the permanent settling of God's people in the Land (Ezekiel 36), the creation of a new heaven and earth (Revelation 21) to name just a few.

In summary, God's Word is said to never pass away even if heaven and earth do pass away (Matthew 24:35) but the Law and Prophets, having been given for specific and limited purpose do have an expiration date that has yet to be reached. It is also certain that these will ultimately be totally fulfilled by Christ "For all the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory." - 1 Corinthians 1:20

"so shall my word (Christ) be that goes out from my mouth; it shall not return to me empty, but it shall accomplish that which I purpose, and shall succeed in the thing for which I sent it." - Isaiah 55:11