In Romans 5:12-21 this is explained:
- Through the disobedience of one man, sin came into existence.
- Consequence was that death "ruled as king", meaning it was inevitable for all people, even for those who had not been disobedient like Adam.
- Death was not a consequence of the Law: it was there before the Law.
- The Law defined "sin", so that there were legally established consequences; and
- 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.