First of all, God is a just God, and will judge everyone righteously (Psalm 7:11). Second of all, God judges people according to how much they have rejected of Him (Hebrews 10:26-31). Third of all, where someone does not know the law, he cannot violate it (Romans 4:15). So, those that try and follow the law, will be judged by it. What then are the people who have never heard of the law of Moses judged by? They are judged by the law that God has placed in their hearts. Romans 2:14-16:
For as many as have sinned without law will also perish without law, and as many as have sinned in the law will be judged by the law (for not the hearers of the law are just in the sight of God, but the doers of the law will be justified; for when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them) in the day when God will judge the secrets of men by Jesus Christ, according to my gospel.
This passage is fascinating because it shows that each human has an internal law that he/she creates, and then BREAKS. Therefore, we can conclude that God judges each man that has not been exposed to the law or the gospel according to his/her attitude toward right and wrong, or better said, we are judged on what we know about God, and how we act upon it. Here is a hint, we always choose wrong. This makes everyone on earth morally responsible for their trespasses before God. Obviously, one that knows much, from much is expected. If you know that Jesus died on the cross for your sins, you will be judged on your action upon that information: either believe or unbelief. And if you hear the truth, and reject it, then a harsh judgment awaits.
This does not mean that all religions will lead to God! Or that if someone keeps his conscience clear he will eventually pop up in front of the pearly gates. Not at all. It just means that #1 all people will be judged. #2 All people have a God given "law" or conscience, for which they know what is right and wrong. #3 That people will be judged according to how much "light" they are exposed to, and how they react to it. Those who hear of Christ are responsible for hearing that information, and how they act upon it.
Here is the curve-ball.
It is not so much a question about how God judges people who have never heard about Jesus, but how God judges people who have rejected Him, and rejected the ways that He has manifested Himself in nature, and in our hearts (Ecc 3:11). The problem is not that people haven't heard about God, but that they have rejected everything (However little it may be) that they have known about God.
Everyone rejects God, and chooses to follow false things. Romans 2:20-25
For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened. Professing to be wise, they became fools, and changed the glory of the incorruptible God into an image made like corruptible man—and birds and four-footed animals and creeping things.
The problem is not that people haven't heard about God, but that they have rejected everything that they have known about God. So, everyone has rejected what they know about God.
Thus, even if you haven't heard about Jesus, you are guilty of rejecting God. BUT! (here is the good news!)
Deuteronomy 4:29: But from there you will seek the LORD your God, and you will find Him if you seek Him with all your heart and with all your soul.
Everyone who seeks God, and places all selfish ambition aside, and has a truly searching, and repentant heart will find the One True God. However, the problem really lies in the fact that no one seeks. (Romans 3:11)
I would not pretend to know everything about this subject! But above all, keep in mind that God is sovereign, and just. God does not want people to be separated from him. Furthermore, there are many stories of Indians being visited by angels, and told the Gospel in their own language. Stories also of people that have never heard of Jesus, but the moment they hear about Jesus, they can identify Him, and even go as far as to say they have a relationship with Him.
God reveals Himself to those that search for Him.
This question really shouldn't bother Christians a lot because we have already responded to the truth that we have been exposed to, and anyone that tried to argue about the people that have not heard the gospel, has clearly already heard the gospel, so he/she need not worry for the reaction of others, only the one for his/herself. Also, we are commanded to "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I[Jesus] commanded you ." We should go, and expose people to the "light" that we follow, so that more and more people can respond to it, and enjoy a personal relationship with our creator.
I would love to hear thoughts! And please let me know if I am mistaken in something!