In the West is it even debatable that christianity is on a decline?
As for the muslim world its more mixed but I do think the younger genereation is less religious than the older ones. It has a long way to go though till we can say its a secularized society.
“Religious-ness” has two contexts. One is for philosophical purposes, where a person agrees with a set of assumptions (aka beliefs).
The other context is as a tribal marker, for practical purposes. In this context, the “religiousness” is not about agreeing with the set of assumptions, but pretending to agree and/or going through the motions of various customs as a signal of how committed they are to the tribe.
In societies where individuals need to rely less on the tribe, it might be posited that there is less reason for people to put on a show for the latter reason, so they can afford to be less “religious”. So if, on average, children of Christians are more likely to be economically independent, then they may tend to being less religious, and children of Muslims, who are more likely to be economically dependent on each other, tend to be more religious.
