This is "lie to join a group" for people who don't believe, and the dishonesty has negative effects on people as well.
The conceit behind it is they think there's a chance you'll believe after joining.
If it's not your thing, it's not your thing, but if 'lying' is really the only barrier, note that a lot of churches actually consider it part of their mission to work with nonbelievers and would take something like "I'm not a believer but I'd like to learn what it's like for you all" (or some other true formulation of your intentions) as a valid form of interest.