Premise: Christians believe that God wants us to love one another and, even prior to that, to love Him; salvation also requires that we accept Jesus Christ as divine. See, e.g., John 3:16-18 and 14:6.
Now, it is commonly argued that it seems very unfair that someone who leads a good life, full of selfless lovingkindness to his fellow man, should be kept out of Heaven and, indeed, sent to everlasting torment in Hell simply because he did not believe in God — or, even worse, did believe in God but not the God Who includes Jesus Christ as part of His trinity. So why would God have such a rule? (And I think it is fair to characterize this as a God-made rule, as opposed to a rule that has to be true whether it was made by God or not, like 2+2=4.)
That seems to me to be a fair question, though it must also be said at the outset that a fair answer is, “Because He said so.” That is, if God demands something — and the evidence is good that He does make this particular demand — then the failure of a human being to agree with the soundness of that demand is no reason for that human being to ignore it. It is, to quote Martin Luther from a different context, neither prudent (since God determines who gets eternal bliss and who gets eternal torture) nor wise (since God has His reasons and He is way smarter than we humans) to ignore God’s biggest commandment.
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And in any event it is also not an unreasonable demand at all, when you think about it. The most obvious reason for it, though not the only one, is that God wants to incentivize godliness, a big part of which is treating one another with love, which most of us would view as a worthy aim. Godly behavior is much less likely to happen if people don’t believe in God, and of course God would want people to believe in the true God, not some fanciful and fictional being that fails to require godly behavior as God defines it and, worse, might legitimize ungodly, even evil, behavior. So God would want to encourage nonbelievers to explore belief: to encourage atheists to explore theism, and non-Christian theists to explore Christianity. And a loving God would also want to encourage evangelism — and Jesus does in the Great Commission — to spread the godliness even further.
He can’t do any of that very well if His rule is instead, Believe whatever you like, so long as you behave properly. We can’t know how to behave if we don’t believe in the right God (indeed, we know that, even when we do believe in the right God, our behavior won’t be perfect). Our reasons for taking a serious, hard look at Christianity will be much diminished if, at the end of the day, belief in it is, so far as God is concerned, strictly optional. And how can we persuade other people to behave well if our own reasons for doing so are idiosyncratic or murky?; indeed, it seems very unlikely that one would be inclined to be evangelistic at all if one were an unbeliever; and how “good” can a person be who has not received the Great Commission?
Far better, then, for God to give us passages like John 3:16-18 and John 14:6.
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P.S. Consider this line of argument: Should someone be allowed into Heaven if he deliberately rejects Christ, despite knowing He existed? If not, what about someone who suspected Christ’s existence but still decided not to accept Him? How about someone who should have or could have known Christ but was just too busy with other things — and does it matter what those “other things” were? Now, how do these different reasons for not accepting Christ differ, exactly, and are these the kinds of lines that we would expect God to draw in deciding who has eternal life? Just asking!
P.P.S. I concede that I have not addressed specifically the hardest question in this area, namely whether/why salvation should be denied to someone (say, in some Third World jungle) who has never even heard of Jesus Christ. Perhaps God makes His, including Jesus’, presence known to such a person in some way before he or she is sent to Heaven or Hell. But the fact that this is just speculation ought to — and I’m sure does — motivate missionaries to those faraway places.