Or in fact used at all by any member of any religion at all.
To me, both “churchgoer” and “person of faith” have pretty Christian specific implications behind them. While many people, myself included, will tend to use the term church as way to reference any building used for religious gatherings, that is similar to how Kleenex or Xerox are used in place of the generic terms, simply because, at least in the West, Christianity is the overwhelming majority. When I’m around more devout Christians, they sometimes even take offense at using the word church being used to describe non-Christian buildings and, in fact, sometimes even at being used to describe a Christian building since, technically, the church is the group of people, not the building where they meet.
Similarly, faith seems to be much more important in Christian theology than it is in many other religions. For instance, I’d have a hard time really thinking of a Buddhist, even a very devout Buddhist, as being a person of faith. Even within the Abrahamic religions, certain sects of Judaism don’t seem properly described as faiths. Perhaps faith has become sort of a catch-all, but I think that’s yet again sort of just taking the use of the term in Christianity and generalizing it.
So, frankly, if you’re going to use churchgoer or person of faith, unless you’re specifically trying to grab other religions, Christian pretty much does the work. Sure, there are plenty of Christians that don’t go to church, but even for those who are and don’t attend church, I would think the general mental image for an average Christian would presume they do.
If you’re trying to get the same for all religions, I don’t think there’s a good single term to grab it. Theist is a pretty weak and inclusive term. Believer is a little stronger, but certain doesn’t get quite to that level either. Spiritual seems to grab a lot of other stuff not specific to religion and doesn’t imply attending services either. If you want to get as generic a term as possible for churchgoer, I think your best bet is something like “religious adherent” since it doesn’t include the Christian specific terms and yet, for all religions that will generally assume some sort of weekly or periodic service attendance, that would be adhering to their doctrines.
If you’re still really going to push for a one-word term, I think “worshipper” might be best. For me, it would exclude a religion like Buddhism, but then you’re getting a little nit-picky. Yes, church services include other parts besides worship, like learning, ritual/ceremony, and community, but it seems to me to be the significant aspect that differentiates attending a church service from, say, a community gathering or a lecture teaching about interpretation of scription, or some generic ceremony.
So, TLDR, your most specific term I’d go with would be “religious adherent” or something very similar. And if you really want a single word, I think “worshipper” is the best you’ll get.
I don’t think you can use religionist. Dictionary.com says:
religionist
re·li·gion·ism
noun
- excessive or exaggerated religious zeal.
- affected or pretended religious zeal.
“God-lover”?
I don’t think “believer” sounds too specifically Christian. Though assuming the OP is in a Western country, probably whatever term he uses will have some people think of Christianity since it’s usually the most common religion.
I’d vote for “believer,” or as a second choice, “adherent.” Or maybe “religious observer”?