The first part of the question is clear, and the answer is clear: Unless Jesus is the victim of grandiose self-delusion or deliberate, blasphemous lying, he is the only way, for he says exactly that (John 14:6). But the second part of the question is not clear. People who have never heard of Christ, and thus have neither consciously accepted him nor consciously rejected him, must also get to Heaven through Christ, for there is no other way. That much is clear from Christ's own words. But it is not clear what is going on in the unconscious depths of the souls of such people. Only God knows. Perhaps they know and love him in the obscure form of a deep, unconscious desire and love.
The game of heavenly population statistics is one that Christ discouraged his disciples from playing. When they asked him, "Are many saved?" he answered neither yes nor no but said, "Strive to enter in" (Luke 13:24). In other words, mind your own business, your own salvation, rather than speculating about others and statistics. God has not told us the answer to this question, for his own good reasons, just as he has not told us when the world will end, another question about which we love to speculate. I think that in both cases we can see the wisdom of not telling us. If we knew when the world would end, we would not be ready at all times for the thief who comes in the night, unexpectedly. If we knew that most were not saved, we would tend to despair; if we knew that most were saved, we would tend to presumption.
What we do know is that Christ the Savior is not only a 33-year-old, 6-foot-high Jewish man, but also the eternal God, the Logos that enlightens every individual (John 1:9). Thus everyone has a fair chance to accept him or reject him, whether implicitly (for all light of truth and goodness is from him) or explicitly. We are not saved by how explicit our knowledge is; we are saved by him. Faith is the glue that holds him fast (or, more accurately, the glue by which he holds us fast, for faith is also his gift).
This is a traditional, mainline Christian position, from the time of Justin Martyr and Clement of Alexandria to the time of C. S. Lewis. It is halfway between the liberal view that one can be saved in other ways than Christ (for example, by good intentions) and the frequent fundamentalist view that it takes an explicit knowledge of Christ to be saved.
The middle view does not detract from the infinite seriousness of missionary work, as the liberal view does. For if we do not know how many children will fall through a hole in the ice and drown, we feel just as much urgency in shouting warnings (and in putting our words into action) as we would if we knew exactly who would die and who would not.
This was published in the Christianity Today book, Tough Questions Christians Ask. Peter Kreeft is a professor of philosophy at Boston College and the author of more than 40 books