According to the Pew Research Center (2012), Christianity has 2.2 billion adherents, Islam 1.6 billion, and Hinduism 1.1 billion. At least ten other contemporary religions claim between 1 million and 500 million believers. Disparities among these religions are not insignificant—some are polytheistic and some posit reincarnation rather than a conscious afterlife. It can hardly be claimed that we all worship the same God. How certain are you that you happened to be born into the society in which the dominant religion (or your parents' religion) is the one that believes in the true God?
Or, perhaps an extinct religion was the true one. According to Harvard biologist E. O. Wilson, humans have invented over 100,000 religions, most of which have become extinct. The most familiar extinct religion in a Western context is Greek mythology. Citizens of the Greek empire were intelligent, literate, and sophisticated. They worshiped Zeus, Apollo, and other Gods, and were as certain of the truth of their beliefs as modern Christians and Muslims are of theirs. If we now think of the Greek religion as quaint and baseless, how might Earth's inhabitants 1,000 years hence regard the religions of today, for which there is no more evidence than for the Gods of the Greeks, Egyptians, and Phoenicians? If it appears obvious to us that Apollo does not exist, how does it not appear equally obvious that the Christian God does not exist? Evidence for each is equally absent.
Everyone is an atheist. Christians do not believe in Zeus. Muslims do not believe in Thor. Hindus do not believe in the Christian God. How do you know you believe in the right God? If you are certain in your belief, how do you explain the fact that most people in the world are as certain as you, yet believe in different Gods than yours? As author Stephen Roberts argues, 'I contend that we are both atheists. I just believe in one fewer God than you do.' —Dan Dana