Bit of a fringe issue with a workaround:

"Research Reinvented" (abbreviated to RR) replaces the default research system with a wider selection of different research activities. As this is effectively a different research system entirely (even if the 'basic research' type is functionally identical to the base game), cult-mindedness is not coded to increase when using the modded research activity, making it impossible to start a new cult with RR active.

A possible workaround is to disable RR until you start a cult, then re-enable RR once you are able to increase cult-mindedness with sermons, though it should be noted that the "Study Successful Cults" research option will not work properly with RR active.