People have been responding in terms of treating psychotic delusions, but that wasn't what came to my mind when I read the question. What about regular old extremism? Is there any role for psychiatry there? Like Whopper says it can be detrimental.
Even following the DSM criteria can be misleading, unless you also apply common sense. For example what if you had a muslim fundamentalist in a small town dominated by fundamentalist Christians--the person may certainly not seem like "them." Also a lot of issues like job discrimination could arise and on the surface the person could meet our DSM criteria for being "impaired" or whatever. Or if a pentacostal moved to, say, some atheist enclave of northern california, and continued speaking in tongues--that shouldn't transform them to "psychotic."
I don't know because I haven't done outpatient psych and the only time you really address "religion" in inpatient psychiatry is when the patient demonstrates "hyperreligiosity" or religious delusions. But I would not actually call that "religious extremism." I would call that mania or psychosis. People also have delusions that they are the President of the US but we don't say that we're treating political fervor when we give those patients antipsychotics. But yes, psychosis and mania show up all the time with "religious" elements. People will go on fasts, they won't drink water, they will stand in traffic reciting bible verses--we see that all the time!
But what about an extremely religious narcissist? Someone who insists their dogmatic view is the only view, or thinks they are especially blessed... Perhaps more experienced residents or attendings could address this.
I think there are some histrionic figures in the public religious realm--TV evangelists, etc.
Oh, and what about converts to extremist cults? I've known a couple people who joined them, including one who later got out and wrote articles about being "brainwashed." The other person I knew never escaped and has since lost all contact with former friends and family. I wonder if any therapy could have helped her... She had features of several personality d/o's.