IMO, religion has three causes: a) being aware of death, b) a desire to explain, c) need for a parent-in-adulthood. Religion is on the decline in the developed world because religion can't explain much of anything that science can't much better. It's explanatory power has been stripped bare of any meaning and therefore open to ridicule for those educated or rational enough to do so. However, a) and c) remain strong as causes and so we should expect religion to remain in some form until they have been eliminated. Simple as that.