Therefore I will give him a portion among the
great, and he will divide the spoils with the strong, because he poured
out his life unto death, and was numbered with the transgressors. For he
bore the sin of many, and made intercession for the transgressors.
How can it say that "he will see his offspring and prolong his days", seeing as he died unmarried? It is because it also says that "He will see the light of life", referring to his resurrection. And since then multitudes reckon themselves his offspring.
"by his knowledge my righteous servant will justify many" - that is "through the knowledge of him, or through knowing him" - for justification comes through faith in Christ which is initiated through knowledge of the gospel of Christ.
"Therefore I will give him a portion among the great" - What is interesting here is that while you would think that Christ should be honored simply for who he is, God honors him for what he did. This is in contrast to worldly leaders throughout history who demand respect simply because they hold a particular position. Jesus deserves honor not simply for who he is but for what he did.
So also for the Christian. It's one thing to be saved and forgiven - to be given a position in Christ - but to be honored by God is something we need to work for.