I think you're seeing a decline because religion has been basically used as a bludgeon for decades now (well in all of history but here in the U.S. increasingly moreso).
It's simply alleged moral cover for discriminating against people, hating them, or in the case of the jihadists, enslaving them, mass-murdering them, brutalizing them, etc.
It doesn't help that 100% of the rage merchants, were you to ask them, would say they're Jesus-loving, religious Christian people.
Not exactly a great recruitment strategy for younger people. Who wants to sign up for this kind of life?
The other thing that's probably sinking it is the increasing rejection of science and embrace of religious-based theories that are, frankly, absurd.