In ways known to himself God can lead those who, through no fault of their own, are ignorant of the gospel, to that faith without which it is impossible to please him, the Church still has the obligation and also the sacred right to evangelize all men.
Some people have never heard of Jesus as the Son of God (if they've ever heard of him at all in most Muslim countries evangelizing by Christians is illegal) or have grown up with tremendous prejudice against Christianity. These are quite different from those who knowingly and willingly reject the truth available to them either by refusing to embrace it or by abandoning it later on.
In 1 Peter 1:20 and other places in Scripture we read that Christ's redemption was planned even before the creation of the world. Peter notes that the revelation of this redemption wasn't given until "at the end of the times," so there is an immense span of time between creation and the time when God's plan of salvation would be fully revealed. What about those who lived before this revelation was given? Are they simply out of luck, without any hope of salvation, because they lived before the time Christ would be fully revealed?
God gave hints from the very beginning of a Savior (Gen. 3:15), but they were only hints and not full revelation. There would have been no reason to give people these clues if their salvation did't depend on the future work of the Savior. But their salvation didn't depend on explicit faith in Christ because he had not yet been revealed to them.
Paul declares in Acts 17 that from the very beginning God's intention was for men to seek and find him. Paul says even those without any direct revelation of God still have his moral law written in their hearts (Rom. 2:14-15) and can know much about God through the witness of creation (Rom. 1:20). God has made it possible for every person to have knowledge of him, which makes faith in him possible. No one is totally "in the dark," and, as Isaiah attests, those who seek God and find him will find mercy and forgiveness (Is. 55:66), two necessary components of salvation.
The first covenant God makes with man after Adam and Eve's fall is with Noah. "Noah was a righteous man, blameless among the people of his time, and he walked with God" (Gen. 6:9). Noah didn't know of Christ, but he did have faith based on the knowledge of God he possessed at the time.
What was the point of giving the law if it couldn't save anyone? Why didn't God just reveal Jesus right then and there instead of waiting another 2,000 years to reveal him? As Paul explains, the Law was a shadow of the reality to come; it was preparatory in nature (Heb. 10:1-3). "So that the law as our custodian until Christ came, that we might be justified by faith" (Gal. 3:24). The law was never intended to be the means by which people could "save" themselves (as many first-century Jews erroneously concluded) but was given in part to show them what God's holy standards truly were, even though they weren't meeting them.
Paul distinguishes between three kinds of people. Those who knowingly reject God receive God's condemnation (Rom. 1:18-21). Those who try a do-it-yourself way of salvation through following the law will be judged by the law and condemned, since they fail to keep the law perfectly (Gal 3:10, Rom. 3:20, Gal. 5:3). Both of these groups of people have rejected the merciful initiative of God's grace in offering them salvation.
There is the third group, which actively seeks God and finds salvation in him (Acts 17 says that these can be Gentiles as well as Jews). They are responding to the initiative God makes in anyone's salvation (John 6:44) by drawing them to himself. Just as people who lived before Christ was revealed didn't have to know about him specifically to receive salvation, neither do people who have lived since the time of Christ have to specifically know about Christ in order to be saved if he hasn't been revealed to them yet.
We are speaking here of those who through no fault of their own have no way of knowing of Christ or have not heard of him in a way that they can clearly understand who he is. If God commanded explicit faith in Christ and baptism as the only ways to be saved even for those who have no way of knowing about Christ then he would be commanding such people to do something they're not capable of doing. This would be unjust and incompatible with his character. God doesn't give us commands that we are not capable of obeying.
Through Christ God has provided the redemption necessary for salvation even to be offered, and that redemption is for all men without exception. Now all that needs to be done is to apply that redemption to each person, which is a way of describing salvation. It is repeated throughout Scripture that we are saved by grace through faith. As Hebrews 11 testifies, this saving faith doesn't require explicit faith in Christ when there has been no revelation of Him.
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