Now we're really get into the meat of the whole thing; the way the Old Testament works its way into the New Testament.
In the Old Testament, the Law was the covenant. If people kept the law 100 % as prescribed by God, they would be saved. But no one can keep the law 100% so they sacrificied animals on a regular basis for what they called "atonement". But the blood of animals can't forgive sin any more than a naked mole rat can (yes, a Kim Possible reference there). The law was unable to be kept perfectly, but that's what God required.
But when Christ came, he fulfilled the law; he never sinned. He died on the cross to pay the price for our sin. When he died and was resurrected, he defeated the power of death and sin. He established a new covenant which simply was "have faith in Christ, and by his grace, you will be saved".
So, a little more about that first covenant, the law. Was it impossible to live 100% by? Yes, and Christ said that if a man has broken part of the law, he has broken the entire law. So, does the impossibility of the law mean it was a failure, a mistake? No. It was planned. As Paul tells us, the law (the first covenant) was a "school master" or "tutor" to show people their sinfulness, to convict them of it, so that when Christ came, they would realize that they can never earn their way into heaven by keeping the law or doing good works, and they would recognize their need for a savior.
I think that any Jew who puts his faith in Christ will be saved. I think that any person who lived before Christ, who had true faith in God had that faith reckoned to them as righteousness, and were accepted by God. That "faith reckoned to them as righteousness" idea is presented in Hebrews.