You really think you can convince him?
Here is what you do, you start talking to your boss: I used to belong to a cult, I left a long time ago, but this guy is a leader there, I just want you to be aware, because they’re really insistent about me going back and they spend their time proselytizing even at work.
Then if he starts, you say, I really don’t want to talk about this, this is a place of business and you have no right to use your paid hours proselytizing here. If he insists or repeats, you file a complaint to your boss. You keep doing that every time, until he gets fired.
Or he is aware of the risk and doesn’t want to get fired, he keeps his mouth shut.