We know from extra-biblical Jewish writings, such as the Talmud, that the person believed that Hades had two compartments. One compartment was hell, where the unsaved went to await judgment. The second compartment was called both Abraham's bosom and paradise.
Now why would believers not go to heaven when they died? Why would they go to paradise? Because their sins had not yet been paid for. They were captives waiting for their redeemer to set them free. Going to heaven would have meant going into the presence of God and as I showed with the Holy of Holies, nobody was allowed to do that. But when Jesus paid for our sins, He opened the way to the presence of God. So the curtain was torn and He took those who were waiting in paradise to heaven to be with the Father.
So all those OT Saints who had never heard of Jesus were taken to heaven and into God's presence through and by Jesus. Just as Jesus said, no one could come to the Father except through Him. So now we go back to the present. Since Jesus is the one who opened the way to God the Father, if God offers salvation to someone who had not heard the gospel, wouldn't it still be true that they got to the Father through Jesus Christ who opened the way? I believe that to be true.
What about the Great Commission?
All people are accountable to God whether or not they have "heard about Him." The Bible tells us that God has clearly revealed Himself in nature (Romans 1:20) and in the hearts of people (Ecclesiastes 3:11). The problem is that the human race is sinful; we all reject this knowledge of God and rebel against Him (Romans 1:21-23).
Deuteronomy 4:29 proclaims, "But if from there you seek the Lord your God, you will find Him if you look for Him with all your heart and with all your soul." This verse teaches an important principle - EVERYONE who truly seeks after God will find Him. If a person truly desires to know God, God will make Himself known.
The problem is, "There is no one who understands, no one who seeks God" (Romans 3:11). People reject the knowledge of God that is present in nature and in their own hearts, and instead decide to worship a "god" of their own creation. Ultimately, it is not the best thing to debate the fairness of God sending someone to hell who never had the opportunity to hear the gospel of Christ. People are responsible to God for what God has already revealed to them. The Bible says that people reject this knowledge, and therefore God is just in condemning them to hell.
Instead of debating the fate of those who have never heard, we, as Christians, should be doing our best to make sure they do hear. We are called to spread the Gospel throughout all the nations (Matthew 28:19-20; Acts 1:8), called the Great Commission. We know that people reject the knowledge of God revealed in nature, and that must motivate us to proclaim the good news of salvation through Jesus Christ.
If we assume that those who never hear the gospel are granted mercy from God, we will run into a terrible problem. If people who never hear the gospel are saved, it is logical that we should make sure no one ever hears the gospel. The worst thing we could do would be to share the gospel with a person and have him or her reject it. If that were to happen, he or she would be condemned. People who do not hear the Gospel must be condemned, or else there is no motivation for evangelism. Why run the risk of people possibly rejecting the gospel and condemning themselves when they were previously saved because they had never heard the gospel?
In the end, we can be confident that God will not judge anyone unfairly. Christian exclusivism (that one must place their faith in Christ in order to be saved) certainly should provide the Church with more than sufficient motivation to evangelize every person on Earth for literally people's eternal destiny hangs in the balance. God has simply not informed us of any back-up plan He may have for those who do not hear the gospel in this life. We must assume that there is none.
CONCLUSION:
I don't claim to have all the answers, nor do I claim to speak for God; but I do think I know His nature because He has showed it to us in His word. Therefore I believe my view is consistent with His nature and with Scripture. No matter what, I know that God is fair and loving to His creation.