It is the usual life cycle of a civilization. A civilization rises when it has a sound religion. This causes a rise in science, literature, etc. But also a rise in wealth and in skepticism. And this undermines the religion that made the civilization rise in the first place, which causes decadence and then decline. In the decline, religions goes first, then morality, then the arts, then science, then wealth, and finally military power which allows it to be conquered. We are quite far along in this sequence.