"Religious fanaticism" refers to extreme and intense devotion or belief in a particular religion. It often involves an unwavering commitment that can lead to intolerant or extremist views and actions. Full definition
He's probably just a man who's mind has been warped by religious fanaticism since a young age. (religion.blogs.cnn.com)
The artist is specifically addressing concerns about religious fanaticism in the US, which is primarily Christian and coming from a very vocal minority of Christians at that. (religion.blogs.cnn.com)