I'm not a religious person, but people, in the end, are all just people.
If you have SUCH a strong personal belief that you feel you won't be at your best, then refuse the session - it's up to you how.
I have had some really wonderful gospel sessions in my life, as a producer, choir director, composer, arranger, engineer, guitarist, keyboard player and, even vocalist.
My opinion is that if the music and the artists are inspiring enough, you might actually get into it - I mean REALLY into it.
If you don't take the gig, you might actually miss out on a valuable experience.
However, only you know yourself well enough to be able to gauge how much of a drag it could end up being.
Hell, I've had gangsta rap artists that were ten times more polite, professional, cool and inspiring to work with than some of the gospel artists I've worked with. I worked with Wu Tang in my studio years ago - they were amazing. And, to be fair, I've worked with plenty of nightmare rock bands and classical and jazz musicians, and OTOH had great experiences with plenty of musicians in all the same genres. It almost seems as if genres don't really or necessarily define the client - sure you can stereotype, but there are SO MANY EXCEPTIONS. (Why am I getting the feeling that the liturgy isn't necessarily the only reason you're being resistent here?)
I've also worked with some really wonderful gospel artists who were an absolute joy.
Still, in the end you NEVER know who you're going to end up getting stuck with.
My basic cardinal rule is: I don't commit to any project until I've had AT LEAST one pre-production meeting with a client, I can usually tell if we're going to hit it off - I've been wrong, but
usually I'm not. The pre-production session usually comprises review of all material and full discussions of all expectations, agreements, scheduling, roles, etc.
IOW, I couldn't ever see making a mistake like not knowing what kind of music I'm recording before I commit. I'm not being critical, just relaying my process. Of course, it took me years to learn that it is ABSOLUTELY IMPERATIVE to get to that level of interview before one takes on a project.
Good luck, and let us know how it turns out - luv to hear the recording when you're done, if you decide to take on the challange.