Nonetheless, nature abhors a vacuum and just about everyone, including the most strident atheist, requires something to believe in. And so the exit of Christianity drew in new faiths, and one in particular, to replace it.
Faith, whether it be called religion, anti-religion or whatever, is a practical requirement for nearly everyone, simply because people aren't prepared to face an overpowering universe and all the uncertainty in it as a lone individual. They require beliefs like a man requires garments in winter.
The people who pushed Christianity out of the West, however, were anti-religionists, and abysmally bad at mythology. They didn't even try to replace the old myths until fairly recently, and poorly at that. And so their new mythology came together haphazardly. It was, in fact, confused and unclear, which gave all sorts of rehashed lunacies an opportunity to plug in.
Perhaps a still greater problem with this mythology is that it made course correction all but impossible. With nothing specified, each new addition had equal standing with had come before: So long as they weren't friendly to the old Christian way, there was no justification for removing them.