Despite the efforts of some high profile atheists in the English speaking world, the majority of the world's 6 billion people (92% according to Adherents.com) still believe in God, or at least follow a religion.
The best figures available (again from Adherents.com, supported by Religious Tolerance.org and others) indicates that the beliefs with the largest number of adherents are 1. Christianity (2.1m), 2. Islam (1.2-1.5m) and 3. Hinduism (0.8-0.9m). Non religious tally about 0.8-1.1m, but half of these believe in God but don't follow a religion, and half are non-believers.
This much is fairly clear (although the exact estimates of numbers vary a little). But who is growing fastest? The best estimates seem to indicate that, of the major religions, Islam is growing faster than the world's population growth, Christianity is growing at close to the world growth rate, Hinduism is static and unbelief is slowly declining. Projecting forwards, it is estimated by some that Islam will pass Christianity as the worlds largest religion sometime in the 22nd century.
However a recent article in the Sydney Morning Herald, quoting historian Philip Jenkins, suggests these projections fail to account for one of the major factors in religious growth - population growth.
In his books The Next Christendom (2002) and The New Faces of Christianity (2006), Jenkins argues that population growth in the growing Christian parts of Africa, South America and Asia together with declining growth rates in many Muslim communities in the Middle East and in Europe, will change this picture.
Christians south of the equator find the agricultural, poor, sometimes oppressed and persecuted world of the Bible, with its un-modern emphasis on spirits and miracles, very familiar, and take to it more easily than sophisticated westerners often do.
Thus he concludes that the Christianity of the "south" will be remain more supernatural, traditional, yet socially radical, than the familiar Christianity of the west. And he suggests southern Christianity, already dominant numerically, will become more and more influential.
At the same time, he notes a dramatic fall in birth rates in some Islamic countries (e.g. from 6 to 2 births per woman in Iran between 1986 and 2000), and suggests this will reduce religious growth rates and lead to a modification of belief and attitudes.