NIV 1 Peter 3:18-22
18. For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive by the Spirit,
19. through whom also he went and preached to the spirits in prison
20. who disobeyed long ago when God waited patiently in the days of Noah while the ark was being built. ....
The historical and orthodox answer is simply this: All men who are rescued from their state of sin are rescued by faith in Christ. Old and New Testaments alike.
The only difference is that those justified by faith in Christ BEFORE HE CAME trusted in the coming provision God would make for sin. The promised provision came immediately after the fall where God promised that from the "seed" of the woman, one would come who would reverse the effects of that fall and crush the head of the serpent, although he would be wounded in the process (cf Gen 3:16-17). Adam's response to this promise was one of early faith in naming his wife "Havvah"(Eve), which means "mother of life." This promise of a coming "seed" was fleshed out thru the sacrificial system (the idea of guilt being imputed to another, that of holiness, etc), the kingship of David, and various other "pictures" of the coming provision for sin. Before Jesus came, some (the OT term was the "remnant") were the "Israel within Israel." They were the children of the faith of Abraham. Augustine said this the best when he said (it was latin so it is not exactly the same, but it rhymed then, too). The New is in the Old, Concealed: the Old isin the New, Revealed. All persons for all ages are justified by faith in Christ. They trusted in the coming Christ, we trust in the Christ who has come.... and is coming again (so we believe).
Hope that helps.