For the sake of argument, let's assume for a minute that the Christian God exists. I think that almost all Christians here in RGT would agree that belief that Jesus is our savior is a necessary condition for salvation. I think that all of those same Christians would also agree that God requiring said belief is perfectly just.
Let's examine some reasons for why people do not believe in Jesus.
1) They've never heard of Jesus.
2) They've heard of Jesus but they are very deluded in believing another religion, and they believe that even questioning their own beliefs would be wrong.
3) They have examined the evidence for Jesus and have come to the wrong conclusion (remember we're assuming God's existence) on accident. In other words these types are honestly searching for truth and have just made a mistake.
4) They have heard the Jesus story, but have never been interested enough to give it more than a passing thought, and never put in the time to study the evidence.
5) They don't believe in Jesus because of some ulterior motive.
We can ignore #1 because I think most Christians agree that belief is not a necessary condition for those who have never heard the gospel.
People in categories 2 and 3 are actually somewhat similar in that they both have come to the wrong conclusion while trying very hard to do the right thing. If a Muslim, for example, thinks that they would be sinning if they studied the Bible or looked into any other religions, can they be blamed for not believing in Jesus? Is it just for these people to be denied salvation because of their deluded beliefs? Similarly, can a category 3 person be blamed for doing their best to find the truth and failing?
Category 4 is a bit tougher. These people have never seriously considered the idea that Jesus might actually be divine. But why should they? To believe that Christianity is important enough to study and find out the truth for yourself, you must first believe that Christianity might be true! In other words, it would not be reasonable to expect a person who is not a theist to inspect the claims of every religion that claims it is very important that they do so. It is rare that a Christian takes the time to thoroughly investigate the claims of Hinduism, even though a Hindi would say that they should because it's such an important question.
That brings us to category 5. I'm not even sure what sort of person would go into this category, but let's just say this category exists and that people in this category to deserve to be denied salvation.
The point of all this is that in order to believe that it is just that a belief in Jesus is a necessary condition for salvation, you must believe that for each of categories 2, 3, and 4, either there are no people in that category, or that it is perfectly just to deny these people salvation. I am aware that there are certainly more reasons for disbelief than the ones I have given, but that does not affect my argument and my previous statement is still valid.
There's my argument, I welcome all attempts to tear it apart.