How can Christianity be true for everyone if it hasn't reached certain parts of the globe?
Does the uneven geographical distribution of Christianity prove that it's not a universal truth? To answer this questions we need to ask a question, and that is what do we mean by truth? A common sense notion of truth is what corresponds with reality. It's what matches up with reality. It's describing the way the world really is. Therefore, objective truth is independent of whether anyone believes it or accepts it or not.
Let me give you an example. It's objectively true that man has walked on the moon. There are some people who do not accept this truth. There are some people that don't believe this truth, they have conspiracy theories about it. Whether or not someone believes it or not or accepts it or not doesn't change the fact of the matter. Buzz Aldrin and Neil Armstrong walked on the moon. Would an uneven geographical distribution of people not believing that these men walked on the moon prove that it's not a universal truth, or it's not an objective truth? Of course not, of course not.
Because truth again is what corresponds with reality. Let's go to Christianity, now it's certainly true that Christianity is not accepted by everyone. There is an uneven geographical distribution about Christianity around the world, but does that mean Christianity is not true? Of course not. If you want to know whether Christianity is true or not, you see if it corresponds with reality. For example, did Jesus really rise from the dead or not?
I want to leave you with one final point. That is there is an assumption in this question that if something is true it will be accepted by the majority of people, by the consensus. I think this is false, especially when it comes to spiritual things. We're told that the natural person does not accept the things of the spirit of God for they are folly to him. To the follower of Christ, we shouldn't be surprised that so many people reject Christianity. In fact, Jesus predicted this. He said enter by the narrow gate, for wide is the gate and broad is the way that lead to destruction and those who enter by it are many.