Don't mention the religion at all. It is not pertinent to your job. Stick to behaviors.
Stick to performance and work conduct.
Work conduct--
-Is he following company rules?
-Is he preaching to people in a manner that violates policy?
Performance--
-As a salaried employee, attendance tracking is irrelvant. Not getting his job done is. Leaving when things need to be done, habitually, is a big issue.
-Is he prepared and productive?
-Does he prioritize and lead effectively?
-Is he competent? Does he possess the knowledge and skills needed to perform his job adequately?
-Does he manage his staff properly?
Quirks and weirdness by themselves are not a reason that makes someone unsuccessful, as you know. You need to find out what else it is and articulate it. Put yourself in the employee's shoes and ask yourself, if my director said/did that to me, how would that affect my ability and motivation to perform?
Once I had a manager who "didn't mesh well with her subordinates", so her boss made her take us all out to lunch. Then the boss called us each privately afterwards to find out how it went, and we gave direct quotes from the manager, said during lunch. It was the nail in her coffin. This woman had major personality issues and couldn't act normal. You could suggest that...