If salvation is through faith in Christ, it's logical to ask how anyone might have had faith in Christ before Christ was ever born. The short answer to this question is that God had promised, or prophesied, that Christ would come, even since the time of Adam and Eve. Before Christ was born, people were saved by having faith in the Saviour who would come as God had promised He would.
In the account of Jesus' birth we read of a man named Simeon. The Bible says...
Now there was a man in Jerusalem, whose name was Simeon, and this man was righteous and devout, waiting for the consolation of Israel, and the Holy Spirit was upon him. And it had been revealed to him by the Holy Spirit that he would not see death before he had seen the Lord's Christ. And he came in the Spirit into the temple, and when the parents brought in the child Jesus, to do for him according to the custom of the Law, he took him up in his arms and blessed God and said, “Lord, now you are letting your servant depart in peace, according to your word; for my eyes have seen your salvation that you have prepared in the presence of all peoples, a light for revelation to the Gentiles, and for glory to your people Israel.” (Luke 2:25-32)
Even when Jesus was a newborn child, here is a man who had been expecting the coming of “the Lord's Christ”. So somehow he knew about the Saviour to come. How did he know? He knew because he read the Old Testament which speaks of the coming Saviour. When he refers to Christ as “a light for revelation to the Gentiles”, he alludes to some specific Old Testament references, such as Isaiah 49:6...
It is too light a thing that you should be my servant to raise up the tribes of Jacob and to bring back the preserved of Israel; I will make you as a light for the nations, that my salvation may reach to the end of the earth. (Isaiah 49:6)
Precisely how much detail he knew and understood is a question for much thought and study, and much of the greater detail that we have in the New Testament was not fully understood. The Bible says:
Concerning this salvation, the prophets who prophesied about the grace that was to be yours searched and inquired carefully, inquiring what person or time the Spirit of Christ in them was indicating when he predicted the sufferings of Christ and the subsequent glories. (1 Peter 1:10-11)
Isaiah also wrote, for example...
But he was wounded for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his stripes we are healed. All we like sheep have gone astray; we have turned every one to his own way; and the LORD has laid on him the iniquity of us all. (Isaiah 53:5-6)
This passage speaks of the substitutionary atonement of Christ... “he was crushed for our iniquities... and with his stripes we are healed.” In the 1 Peter passage, it tells us that those prophets like Isaiah were “inquiring what person or time the Spirit of Christ in them was indicating...” God has slowly unfolded His plan of redemption throughout history. Would they have understood this passage to refer to a substitutionary atonement? In the time of David, the first king of Israel (a few hundred years before Isaiah), David wrote:
Sacrifice and offering you have not desired, but you have given me an open ear. Burnt offering and sin offering you have not required. Then I said, “Behold, I have come; in the scroll of the book it is written of me: I desire to do your will, O my God; your law is within my heart.” I have told the glad news of deliverance in the great congregation... (Psalm 40:6-9)
In the Old Testament, the Israelites would offer sacrifices to God for the atonement of sins. But here we read “Sacrifice and offering you have not desired...” The purpose here is to identify that those sacrifices and offerings really point to a true and perfect atoning sacrifice. We see the Psalmist has some grasp of the gospel as we understand it “post-New Testament”; that sacrifices and offering don’t save a person; that the law is a matter of the heart; and that this concept is a the “glad news of deliverance”. In context, he’s talking about deliverance from sin (see v12-13). The sacrificial system of the Old Testament itself spoke of Christ, and we find that system in practice as early as Genesis 4 when Cain and Abel, the sons of Adam and Eve, offered sacrifices to God. God's instructions given to Adam and Eve regarding sacrifices are not recorded in Scripture, so it would seem that at this early stage Adam and Eve had received information from God which was not written down. We may be unclear at times as to how much those Old Testament believers understood, but we know that sufficient revelation was given for them to put their faith in the right thing... that God is our Saviour for we cannot save ourselves.
Perhaps more on this theme tomorrow. Until then...
If you believed Moses, you would believe me; for he wrote of me. (John 5:46)