apologies in advance, as I am assuming the answer may already be in this thread, but nonetheless, here it is.
This is my first transfer of a save between ck3 and eu4. In CK3 I, for the heck of it, created a custom religion, which was ported to the EU4 game. Which is all fine and dandy. But when the reformation event was triggered and I looked to see if I could join in the fun, it said that my relgion was not allowed to convert. So I meddled with some debug console commands, but it did nothing for me. Even triggering the event via the console into my nation did not help.
The wiki does not say anything about limitations for custom created religions in CK3. Mind you, the one I created still followed Christianity as its basis.