Paul preaches salvation by faith, not works, to the Jews by referencing Abraham in the Old Testament.
Paul encourages practicing righteousness, but not as a means to salvation, that's by grace.
We learn about the grounds of Gods judgement upon sinners and mercy upon the righteous.
Paul exposes the deeds of the flesh and its wickedness, and God's wrath on unrighteousness.
Jesus' identity was shown in all the OT, proven by resurrection and His Presence in us gives us Peace.