I vassalized Red early in the game. So we shared the same religion.

I am able to choose all the tenets, so now I'm sitting at max.

Then Red rebels. I fight him and win, let him be free. But I go to check my religion and he is the religious leader! How did that happen? He was my vassal the entire game?

He built more Holy Sites than you (whenever your Religion gets a new tenet, you need to build the new holy site right away to maintain leadership)

Ok thanks, that must be it. I didn't build any holy sites and I saw he had 1. (I didn't built because the Celts' building already generates faith so I didn't think I needed more). At least I was able to choose the tenets and pick the holy site building before it happened.

I'll just have to check there are no downsides - am I getting the benefit of all my tenets since it should still be my state religion.