The apostle Paul througout the book of Hebrews, talks a lot about the relationship between the Old testament Sanctuary and the Heavenly Sanctuary. And in Galations he talks about the Israelites and Spiritual Israel. It is his claim that we, the Gentiles, have inherited the right to call ourselves "Spiritual Israel".
6So also, “Abraham believed God, and it was credited to him as righteousness.”a 7Understand, then, that those who have faith are sons of Abraham. 8The Scripture foresaw that God would justify the Gentiles by faith, and foretold the gospel to Abraham: “All nations will be blessed through you.”b 9So those who have faith are blessed along with Abraham, the man of faith.10All who rely on works of the law are under a curse. For it is written: “Cursed is everyone who does not continue to do everything written in the Book of the Law.”c 11Now it is clear that no one is justified before God by the law, because, “The righteous will live by faith.”d 12The law, however, is not based on faith; on the contrary, “The man who does these things will live by them.”e13Christ redeemed us from the curse of the law by becoming a curse for us. For it is written: “Cursed is everyone who is hung on a tree.”f 14He redeemed us in order that the blessing promised to Abrahamg would come to the Gentiles in Christ Jesus, so that by faith we might receive the promise of the Spirit.
We know that the law is a curse, but that is not because the law isn't perfect, but that we are simply sinful beings who by ourselves are not capable of living by it. The interesting thing about the law and salvation is this, if we could keep the law perfectly, we would not need a messiah or salvation! (that is the point of this)
I would suggest that Paul telling us that we are now part of the same set of covenants that were given to the Jews "they will be my people and I will be their God". We, just like Abraham, are saved by the very same grace afforded to him through our faith in the redemptive power of the death of Christ. This did not change between the two testaments (the two witnesses) of the Bible.
The only difference between the Old and New Covenants is this:
In the new covenant God says "I will write my laws in their hearts and on their minds" ....it is now God who takes upon Himself the responsibility of ensuring the pathway to salvation is given to us. Prior to that, it was the people (Israelites) who made the promise...and they failed miserably at that so God chose a different option in light of the failure of the people. This does not mean God failed...he almost certainly knew that the Israelites would fail to live up the original covenant, but he gave them the option to try anyway.
I disagree that we can create two separate forms of worship here, one group who follow the law, and one group who do not follow the law (its black and white, you either follow the law or you don't) ...there is but one God. How then do we support the claim that He has made a set of rules for the original "chosen people" and a different set of rules for us (spiritual Israel)? It is inconsistent that One God makes two sets of rules for those who are saved through Christ. Are we saying that all those who died prior to the Cross are not saved by faith? Paul very clearly states, Abraham certainly believed he was saved by faith, he was saved by the death of the exact same messiah that we are saved by!
According the to claims of modern Christians, Abraham could not possibly have known of the new covenant. This means he could not have possibly known that the old one was going to fail. So to me this means that Abraham is saved in the exact same manner as we are...by Grace through Faith and that even for Abraham, any attempt to be saved through the law and his own works were, as James wrote, "but filthy rags"!