Are you kidding me? Christianity has about 2.2 billion followers worldwide (which is almost 1/3 of the entire world) and it's still spreading in Africa, Oceania and some parts of Asia. Atheism (Agnosticism to be more precise) is only on the rise in the countries that were never much christian to begin with really...(Scandinavia, balitc countries and lately UK)
EDIT: Even if Christianity were to dwindle in the West, its still quite strong in South America, Eastern Europe, South Africa and some parts of Asia. Then there are also Islamic regions where atheism is almost non-existant, as well as hindusitic regions. The only place irreligion has a chance to take over is Western Europe and maaaybe USA but that could take couple of centuries







