There are occasions when I have discussions with religious leaders (clergy and otherwise) of various faith groups. Titles such as reverend and rabbi, are generally applied to such individuals. My question is less about their "official" titles, as it is about personal address. Is it impolite not to use such honorifics as "father," "your grace," or "pastor" in such cases as "Excuse me, (title) I have a question . . . ." As one not necessarily of the same faith background these seem out of place to me. Should they be used anyway for politeness sake?
This answer is based on experience in the US. Cultural norms in other places can differ.
Context matters. If the person is your religious leader, then presumably it's normal and expected for you to use the title. (It is in my religion; I don't presume to speak for others.) If he's not your religious leader but the setting is religious (for example, an interfaith gathering) then (a) follow others' leads but (b) using titles probably helps everybody understand who's who and won't be seen as personally accepting that person's authority. (If a particular title is problematic for you personally, you can sometimes avoid using any title/name at all, or use a generic honorific like "sir".)
If the setting is completely secular and it's a passing encounter, don't fret over it -- the person probably doesn't expect you to use the title, but it's ok if you do. (This assumes you even know; a Catholic priest's white collar is pretty clear, but not all religious leaders have such clear indications.) The client whose computer you're fixing or the Realtor showing you a house doesn't expect more than "sir", a name, or a polite nod.
If the setting is secular and it's an ongoing relationship, then as other answers have said, you should ask the person what he prefers to be called. If using his title makes you uncomfortable, you can politely explain that and ask for another way to address him. (That said, when in secular settings I've asked doctors, priests, and rabbis how they prefer to be addressed, they've always just used their names, not titles.)
There is no specific rule of etiquette here. In general, we try to refer to people in whatever manner they would like, so a reasonable action could be to simply ask the person about their preference.
However, if you are uncomfortable using an honorific, it is also perfectly reasonable to make that known. "As someone not of your religion, it makes me uncomfortable to address you as __(title)__. Is there some other way I can refer to you?"
Be sure to thank them if they are accommodating. It's all about trying to find a way forward together that keeps everyone comfortable.
At my current job I deal with this on a regular basis. It seems to vary considerably by tradition, denomination, faith, and individual preference.
The way I usually handle it is to omit the personal identifier completely. As in:
"Excuse me, I have a question..."
If it's someone you're going to have an extended interaction with, you might as well ask what their preferred title or identifier is. As a general rule it's better to ask than to guess and get it wrong... Some people get very touchy about using the wrong title in religious contexts.
Usually if they have a preference, you'll get that by simply introducing yourself using your name.
Generally speaking, it's better to use whatever they prefer to be called if you're going to address them by name. It's not specifically endorsing their religion, it's just a general courtesy. Think of it in the same context as referring to someone as Doctor, Mister, Miss or Missus.
I would use a religious title with a religious leader in a religious setting such as a service at a place of worship, either the person's "home" church/temple/synagogue, or at interfaith event. That just shows respect for his/her leadership in that context. Similarly, I would address a doctor as Dr. in a hospital, or an academic as Professor in a university.
In a secular context, I don't see the need to use a religious term. In those settings, the person is as much of a "layman" as you are.
This is a very interesting question. I would argue that it is generally understood that using a title does not involve endorsing the religion. For this reason I would generally suggest just using the title. If your reluctance is related to the normalisation of religion, I would suggest that you would be in a better position to change people's minds if you showed them respect such as by using their adopted title.
At the same time, I have to admit that I would be reluctant to call a cult leader by their adopted title because I feel that the normalisation of these cults is much more dangerous than any potential further normalisation of mainstream religions (which are already normalised anyway). Some might argue that I'm trying to draw a somewhat arbitrary line and I think this criticism is fair to an extent. Nonetheless, even if this position isn't perfectly principally justified, it still seems like a somewhat useful heuristic (in addition to being socially defensible).
TLDR: It depends on whether people consider the group a cult or a mainstream religion.