I used to work for a church in Minnesota that adopted a theology for a while called "dual covenant theology" (think John Hagee) which basically says the Jews who hold to the law don't have to believe in Jesus Christ to be saved.
Well, I knew Jesus said "no man comes to the father but by me" so I set out to prove that there was not two covenants, but only one.
I started with a cursory search of the word "covenant" in the Bible. One of the first passages that came up was Galatians 4:
Gal 4:24 This is allegorically speaking, for these women are two covenants: one proceeding from Mount Sinai bearing children who are to be slaves; she is Hagar.
Immediately, I knew I was wrong. I had to read the context to know if this was true. I read the whole of Galatians. I read and re-read Galatians 3. God began to illuminate it to me and show me some things that blew my mind.
First, that God himself preached the gospel to Abraham (Gal 3:8), and when Abraham believed the gospel, this was what counted as righteousness. It wasn't simply that God said anything and Abraham believed anything God said - but Galatians 3 makes the case that God preached the gospel of Jesus Christ to Abraham and it was specifically believing the good news of the gospel that made him righteous. Second that this covenant God makes with Abraham - the covenant of faith - came 430 years before the other covenant - The Law. So the Law has nothing to do with salvation by faith, and salvation by faith exists apart from and outside of, and independent of The Law. This sheds light on Romans 4, 7, 8 - and really all of the book of Romans. In fact, this passage helped to show me the purpose and function of The Law in the life of the believer is as a teacher to bring us to Christ. The law is a physical lesson about God and how we relate to him spiritually. When the Bible talks about how slaves are treated, it's talking about US who were once slaves to sin, for example. I also came to understand through Galatians 3 that we INHERIT the righteousness of Christ as a birth-right once we are born of the kingdom of God. Having the same faith as Abraham doesn't save us - it makes us children of Abraham (this is what John 3:3 means by "born again" see also Gal 6:15 and 2Cor 5:17) and children of God. Then, the children of Abraham inherit the promise of God that Christ would take away the sins of the world and inherit Christ's righteousness.
Another revelation of scripture I've had was the meaning and purpose of circumcision. Circumcision was instituted when Abraham was made righteous as a sign of that righteousness. It is a shadow and reflection of salvation by faith. The Word says we become circumcised of the heart. I came to understand that circumcision is (again) a physical reflection of spiritual truth. Like the outer flesh is cut away and the inner member lives on in circumcision, so we too have a righteous, born again inner man that will survive the destruction of our bodies (Rom 8:10, 1Cor 15).