What major world religion has the greatest number of adherents today?

In terms of membership, Christianity has more members than any other religion, although Christianity is divided into many different branches, and is not a unified religion. Islam is increasing in the number of members, and may overtake Christianity, although like Christianity, Islam is also divided into different branches, which have often been in conflict with each other. If we are to look at specific sects, Roman Catholicism would still be the religion with the largest membership, making it the major religion. There are other criteria by which we might judge some other religion to be more important, however. For example, although Judaism does not have a large membership, it is also ancestral to the two largest religions, Christianity and Islam, neither of which would have existed without Judaism. One could argue, therefore, that Judaism is the major religion. And of course, everybody believes that their own religion is the most important one, because it is the true religion. Truth, after all, is more important than mere membership numbers. One person who knows the truth counts more than a thousand deluded fools. So there are lots of different ways of looking at this question.