What an arrogant jerk. Now he tells folks how they should believe?
Maybe he meant truly "intelligent" people always strive to keep an open mind.
Or, maybe he meant an evangelist should maintain an appearance of open-mindedness to avoid appearing arrogant.
I really don't know what he meant. But, if it's either of the above, I'd disagree with him.
Your quote does not support your satement that he is telling people what to believe. I read the quote as, colloquially, “believe what you want but don’t be an a$$hole about it”.