In general:

Religiosity is correlated with fertility, the most extreme example being 'quiverfull' people having 8 kids each, with Mormons in a close second.

Religiosity is about 50% genetically heritable, and also mimetically heritable, the extent depending upon the situation.

The secularisation of Europe might have gone as far as it can go, while if anything the US seems to be getting more religious. In the long run, won't genes win out?

Therefore, it seems likely that the world is going to keep on getting more religious. And I'm sure we are all aware that exponential growth curves can cause very rapid changes. Trying to put an exact time-frame is difficult, because of immigration, questions of how long communities can remain isolated from the rest of the country, positive feedback where immigrants vote for more immigration, negated feedback from backlashes, birthrates decreasing in a demographic transition, and so forth.

I did a calculation and decided that within around 100 years many secular countries would be run by religious fanatics, and then I read that the quiverfull movement has around a 20% retention rate. Of course, given exponential growth that doesn't buy all that more time.

The problem isn't that ISIS take over. They don't have the weapons, they don't have the numbers, they don't control any tank factories. The worry is that in 2100 or 2200, if for some reason the singularity hasn't happened, fundamentalist Muslims are a democratic majority in France and evangelicals are a majority in the US, and now there is a far more serious threat than that of ISIS, and the question of whether, with the technology of 2200, the US can disable France's nuclear weapons in a first strike is raised.

Obviously that is just one hypothetical. But as the average religiosity rises, and when both Islam and Christianity have a serious history of violence, it seems likely to end in disaster, if baseline humans are still the dominant force at that point in time.