As a Christian, I share a mission to go out and make disciples. We are the heralds and the mentors in the Christian life. It is only through our willingness to talk that the Gospel is spread, as Paul observes in Romans 10:14.
However, going back to the early days, there are wide groups of people who have never heard of Jesus, or even of YHWH. Paul in particular may have spread the Gospel around the Mediterranean, but northern Europe was still unreached until a century later. Much of Africa and Asia were not reached until later during the Middle Ages, and the Americas were completely isolated for centuries. Even today, we have entire groups that know nothing about Jesus, and countless generations have died without being introduced.
Of course, the obvious objection here is that these people are unfairly denied entry to heaven simply because they have not been told about "the right way." However, I believe that the reason they are denied entry to heaven stems from something much deeper.
At the end of the day, there are two truths available to everyone that every man, woman, and child has rejected. The first truth is that there is an intelligent designer. The principle of Cause and Effect demands that something beyond the universe, something that is by definition supernatural, began the universe. The Law of Causality insists that the universe cannot be self-creating. Furthermore, the general beauty, elegance, and sophistication of what was created is evidently not random. Everything from a perfectly-balanced solar system made to support life to a stable water cycle, from symbiotic relationships to the engineered complexity of single-celled organisms, speak to a designer. Random chance did not produce all of these neatly functioning and cooperating systems. Americans in particular have spent the last century trying to convince themselves that the universe is indeed self-creating, and that everything around us came into being by random chance. However, anyone who is actually interested in merely thinking about it can see that much of it is nonsense. Nevertheless, most people in our culture today refuse to think critically about this, and willingly believe what they are told. Secular humanity in particular insists on denying the existence of The Creator. If they refuse to acknowledge even the very basic truth of the existence of the supernatural, what is the point of them hearing about what he has done?
The second truth, the more widespread and serious one of the two, concerns the matter of the law. Now, different cultures do have some different ideas on specifics of the law, but everyone of sound mind understands that there is right and there is wrong. Everyone has a conscience. The core problem is that every man, woman, and child has at some point ignored this conscience and done what is wrong. No one, outside of willing self-delusion, can deny that they have gone against their conscience. Even if many of us were not given specifics on everything God desires of us, we all still have that core understanding embedded into us. We all have gone against this understanding, and thus, all, without exception have sinned. When these people are faced with God, even if they do not fully understand the triune being that stands before them, they will have to admit that they have given into sin.
Of course, Jesus provides a way out of that, but countless people have and will live their entire lives without ever hearing His name. However, humans are not entitled to receiving salvation. Ephesians 2:8-9 observes that "By grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast" (emphasis mine). We did nothing to earn salvation. In fact, our sins declare that we deserve nothing but rejection from God. However, God decided to give some of us a chance. He is under no obligation to give everyone or even anyone a chance. The fact that he is acting at all is a gift freely given. No one has a right to it. As such, any complaints about God not being "fair" with this approach tend to miss the point.
On the other side of this, people who take pride in their current standing with God also miss the point. Paul declared that "the unrighteous will not inherit the kingdom of God" in 1 Corinthians 6:9, but then declares two verses later that "and such were some of you." He put it even cleared in Romans 3, saying "For there is no distinction: for all have sinned and fall short of the glory of God, and are justified by grace as a gift," (emphasis mine) and "The what becomes of our boasting? It is excluded." Church-goers (myself included) frequently suffer from the temptation to act high and mighty, but forget that we did not start off any different. God actively and independently chose to give us a wonderful gift of a renewed relationship with Him, and we did nothing (both by capacity and confirmed by active choice) to deserve this. It is only in pride that we can have this attitude. We can get frustrated by how everyone around us insists on denying the signs around and within us, but we ourselves certainly have done that. God showed us great patience. Perhaps we should take His lead and show the same patience to those around us. Perhaps then, when we lay down our pride and stop pretending that we are on a higher horse, will those around us begin to listen.