In the broadest sense, “the law” means everything in the Word, as we can see from the following passages,
Jesus said, “Is it not written in your law, ‘I said, “You are gods”?’” (John 10:34, in reference to Psalms 82:6)
The crowd replied, “We have heard from the law that the Christ will abide forever.” (John 12:34, in reference to Psalms 89:29; 110:4; Daniel 7:14)
To fulfill the word written in their law: “They hated me for no reason.” (John 15:25, in reference to Psalms 35:19)
The Pharisees said, “Have any of the leaders believed in him? Only this crowd, that does not know the law.” (John 7:48, 49)
It is easier for heaven and earth to pass away than for the tip of one letter of the law to fall. (Luke 16:17)
“The law” in these passages means the entire Sacred Scripture.
To say that the Lord fulfilled all of the law is to say that he fulfilled all of the Word, as we can see from passages where it says that the Scriptures were fulfilled by him and that everything was brought to completion. See, for example, the following.
Jesus went into the synagogue and stood up to read. He was handed the book of the prophet Isaiah. He unrolled the scroll and found the passage where it was written, “The spirit of the Lord is upon me, because he has anointed me to preach the gospel to the poor, to heal the brokenhearted, to proclaim release for the bound and sight for the blind, to preach the welcomed year of the Lord.” Then he rolled up the scroll and said, “Today this Scripture has been fulfilled in your hearing.” (Luke 4:16–21)
You search the Scriptures, and yet they testify of me. (John 5:39)
. . . that the Scripture may be fulfilled, “The one who eats bread with me has lifted up his heel against me.” (John 13:18)
None of them is lost except the son of perdition, so that the Scripture may be fulfilled. (John 17:12)
. . . so that the word that he said would be fulfilled: “Of those whom you gave me I have lost none.” (John 18:9)
Jesus said to Peter, “Put your sword in its place. How then could the Scriptures be fulfilled, that it must happen in this way? All this was done so that the Scriptures of the prophets would be fulfilled.” (Matthew 26:52, 54, 56)
The Son of Humanity is going, as it has been written of him, so that the Scriptures will be fulfilled. (Mark 14:21, 49)
In this way the Scripture was fulfilled that said, “He was numbered with the transgressors.” (Mark 15:28; Luke 22:37)
. . . so that the Scripture would be fulfilled, “They divided my garments among them, and for my tunic they cast lots.” (John 19:24)
After this, Jesus, knowing that all things were now accomplished, so that the Scriptures would be fulfilled . . . (John 19:28)
When Jesus had received the vinegar, he said, “It is finished,” [that is, fulfilled]. (John 19:30)
These things were done so that the Scripture would be fulfilled, “Not one of his bones will be broken.” And again, another Scripture says, “They will look on the one whom they pierced.” (John 19:36, 37)
There are also other places where passages from the prophets are cited and it does not also say that the law or the Scripture has been fulfilled.
As for the whole Word having been written about him and his having come into the world to fulfill it, this is what he taught the disciples before he departed:
Jesus said to the disciples, “You are foolish, and you are slow of heart to believe everything the prophets said. Was it not necessary for the Christ to suffer this and enter into his glory?” Then beginning from Moses and all the prophets, he explained to them in all the Scriptures the things concerning about himself. (Luke 24:25, 26, 27)
Jesus said to the disciples, “These are the words that I spoke to you while I was still with you, that all things must be fulfilled that were written in the Law of Moses and the prophets and the Psalms concerning me.” (Luke 24:44, 45)
From these words of the Lord we can see that in the world he fulfilled all of the Word down to the smallest details:
Truly, I tell you—until heaven and earth pass away, not one little letter or the tip of one letter will pass from the law until all of it is fulfilled. (Matthew 5:18)
From this we can now see clearly that to say that the Lord fulfilled all things of the law does not mean that he fulfilled all of the Ten Commandments but that he fulfilled all things of the Word.
from The Lord, Sections 11, 12