Well, in a purely deontologic moral system, the beliefs are like "Don't torture people, torturing people is bad". That is, there is a list of "bad" things, and the system is very simple: You may do thing X if and only if X is not on the list. The list is outside the system. In the same way as consequentialism does not provide you with what you should place utility in, deontology does not tell you what the list is.

So when you look at it like that, what the charismatic priest is doing is not inside the moral system, but rather outside it. That is, he is trying to get his followers to change what is on their lists. This is no different from a ice cream advertisement trying to convince a consequentialist that they should place a higher utility on eating ice cream.

To summarize, the issue you are talking about is not one meant to be handled by the belief system itself. The priest in your example is trying to hack people by changing their belief system, which is not something deontologists in particular are susceptible to beyond anyone with a different system.