The Old Testament is a testament to the old covenant that God made with the nation of Israel. In Jesus, we who are Christians, understand that God has initiated a New Covenant. This new covenant provides a new way for God to connect with humanity in and through Jesus Christ. The Cross is the key to all of this. And Paul shows that this religion that "goes way back", as you put it, is NOT the same as that of the Old Covenant, which was based on the law. No, the New Covenant is entered into by faith, not the keeping of the Law.
Thus there are some things that have always been true with respect to God and remain so, that "the righeous live by faith" is one of them. But Christianity is a new thing that God has done in Jesus Christ. We believe that Old Testament saints looked forward by faith to the day when God's anointed, the Christ, would come. And thus by faith, they were still saved in and through the work of Christ on the cross. But the religion they practiced was not Christianity. There is a difference, and if you haven't yet understood that difference, then, to paraphrase an earlier post, I'm afraid you just don't get it.
