I tell the plain truth: there are no gods, and I can prove it.

I usually assert something on the lines of I value truth (things that can be demonstrated) over faith (belief without evidence) I find asserting a faith based claim as fact a dishonest position.

As there is no testable evidence, nothing demonstrable, to confirm that a god exists in the natural (real) world, all god assertion are founded in faith (belief without evidence). As I value truth over superstition, I do not share you belief in the supernatural; or things that are not of the natural world or simply stated (not real).

On the rare occasions that has come up I say that I prefer not to discuss my private beliefs and change the subject. It's never failed to work for me, although I recognize there's no guarantee it will always work.

Also if it's a work situation I'll often add that I don't think it's professional to discuss such things. Because I don't. That is also a dog whistle that suggests that if they persist I reserve the right to regard it as harassment.

When I was closer to evangelical culture I tended to feel obligated to provide Too Much Information and full disclosure, I was way too forthcoming and earnest. I've gradually figured out that it's none of their business and I have zero obligation to describe, much less explain or defend, my personal beliefs. I don't owe that to anyone other than my immediate family; for all others it's generally inappropriate intimacy. While for some liberal believers it may simply reflect curiosity, for a fundamentalist it can't serve any possible purpose other than a source of gossip, backbiting or proselytization.

Fundamentalist Christian culture is very intrusive in this regard, they love to get up into each other's business and "check up" on each other and they're so used to it they don't realize that it's inappropriate and impertinent. Miss a church service and it's "where WERE you last Sunday", as if you have to account for it. And in their culture, you actually DO. I no longer participate in that.

I just say do you really want to know or are you just looking for a reason to preach at me?
I they have even a modicum of honesty left in them they just leave it at that, if not and they do start to preach I remind them there is one of the ten commandments that says not to lie so, if they are right, I'll be seeing them in hell.