A simple summary:
- Before the cross, faith + keeping the law was necessary for the Jews to be saved, Gentiles were totally left out unless they become Jews
- After the cross, faith in Jesus's dbr is all that is required for both Jews and Gentiles to be saved
You cannot keep the law that has pronounced judgement as to the letter, death. There was no work any man could do to get himself saved . Faith is a work that Christ works in us to both will give us his desire and then empower us to do it. Men in the old testament were saved by hearing the gospel as it worked in them.