I say I'm an aetheist and they, um, generally know what I mean .
I generally go on about man-made religion and how humans seem to need to believe in something bigger than themselves/that there's a point to it all/that death isn't the end etc.
I do feel a bit embarrassed talking about it with most church-goers (e.g. my MIL) because I'm kind of saying, you poor deluded fools. But hopefully not so bluntly .