Well there is 1.5 billion christians, 1 billion Muslims, 0.8 billion Hindus, 0.5 billion Budahsim so
1.5+1+0.8+0.5 = 4.1 billion
World population almost = 6 billion
People donot believe in God = 6-4 = 2 billion ( not accurate but almost that number)
Interesting statistics, but they do not include millions of local religions around the world. Also, pure Bhuddism is an atheist philosophy, though many religious people are also Buddhist, and it has become mixed in a few places.
However, the question used the word "God" (with a capital "G") which usually means Yahweh - the Christian god.
In which case, we can say that of a world population of 6.7 billion, there are:
2.1 billion Christians
1.5 billion Muslims
14 million religious Jews
which makes ~3.7 billion believers in Yahweh, and thus ~3.1 billion non-believers.
Non-religious people make up the third-largest slice of the world's population with 1.1 billion. The real number is probably bigger, because many people who live in theocracies or intolerant areas will pretend to be religious.
My stats came from adherants.com (link below).
Actually according to recent calculations there are 1.7 billion Muslims........not trying to be arrogant or all I am just stating what I know.
Nicely reasearched answer though!