What percentage of the world's population are not Christian?

As it is hard to exactly measure religious affiliation, and as many organizations have incentives to exaggerate their membership, it is hard to quantify well.

Christianity is still generally thought to be the most popular religion in the world today. Best estimates is that around 2 billion people consider themselves Christian, though obviously not all belong to a specific religion or church. Thus, out of a total world population of 7 billion, this means that about 71% of the world is non-Christian.