Actually fascism tends to be secular. The authoritarianism is derived from an extreme version of nationalism. The authoritarianism in theocracy is derived from religion. Fascism ≠ authoritarianism. There's a lot of other authoritarian ideologies out there. Marxism is another example.

The the majority of large, brutal facist and authoritarian regimes of the last hundred or two years were secular: Nazi Germany (paid Christianity lip service near the beginning but that fell off as some churches opposed and loyalty to the state became more important), the USSR, Mao's China, and other smaller non-USSR communist regimes that committed massive atrocities (such as the Khmer Rouge)