Even in percentage terms, the atheists populations are increasing rapidly in developed countries(this is mainly the west, and some arab countries maybe exceptions to this ). Western Europe is highly atheistic, and the younger generation in the US is at around 30 percent in the 'atheist//agnosts/don't care' brackets. Source -Pew research.
Worldwide religious population growth and percentages may increase though, but most of these countries are weak and powerless, including the ones in Africa, Latin America and the Indian sub continent. Also, China is highly atheistic, Turkey is halfway there.
The numbers are changing fast, realise that a couple of centuries back, there were almost no atheists. Now there are massive numbers of them.