It is not required, but it does make it easier. When you first ask someone to try and become your religion you get 4 options on what pages to put the bible on, One about a Warrior, One about chairty, One about philosophy and one about Rulership.It is recommended you be an independent ruler to do this. I'm secretly a Cathar of the Catholic heresy and I'm trying to recruit people but it's proving quite tough to invite anyone at all. Indeed, if you continue developing a character's Intrigue skill (or stat), they can be the deadliest. As mentioned in our official review of Crusader Kings III, the revamped Intrigue system has a lot in store for those who want to sneakily dispatch their rivals.
Crusader Kings III: Intrigue guide – Schemes, plots, hooks, and secrets.