I always tell so they know not to bother me with religious doctrine. If they don't want you around anymore they are not worth your time. Telling you are not a believer can actually get a lot off pressure off you. You don't have to feel uncomfortable anymore when certain topics come up. You can give your opinion freely.

I don't really have a problem telling them , sometimes it can lead on to some very interesting conversations.

Inspired them through the ideas put forward in the discussions

I live in the bible belt, and for the longest time have hidden my atheism, when people start talking about Jesus.
I still keep my mouth shut, unless directly asked. "You believe in Jesus, right?" "--No. I'm an atheist."
I don't get asked much, so I don't have to out myself much. Most Christians assume that everyone agrees with them, and most of the time they don't expect, or want confrontation any more than I do.

"Oh, I don't believe in your god, but thanks"
I don't believe in anybody's god, but it's nice to add that extra little tweak to make them think, even fleetingly, about the idea that other people believe in different gods.

I don't know that this is hugely helpful but I think it comes down to what your comfortable with, if you know the people you want to say it to then you're best placed to judge how they would take hearing it and whether or not you need to wrap up the statement in anything else to soften it.

I never tell them

I sport many ill religious tees. As religious wing-nuts have begun an assault on our secular commons and me a studied theologian of hermeneutics, I enjoy Socratic dialogue. The look on their faces when they see reason coming into the picture, as I quote the Bible and say, 'I hate what you have done to my Jesus.'
As an atheist this blows their mind. I tell them I'm here to rescue the Jesus I admire from the eternal Pharisees/religion & Gov.

I tell them directly that I have never accepted Jesus as my savior; nor do I subscribe to any other religious teachings. I mention that scientific method suits me; and therefore anything less is merely supposition or opinion.

It usually starts with a laugh, or a side-eye that lets them know I'm skeptical about what they're telling me. Then I just say it, "I don't believe in that stuff."

I’m from the Philippines and majority of the population here is religious... i just simply say “I’m an atheist”

I just tell people that I am an atheist and of course they try to convert me every time I tried to be as polite as possible and tell them that I have no interest in their imaginary Sky Daddy

I usually just tell them if it comes up in conversation. I do enjoy some of the looks and comments I get.

I CANT and it is smart of me to not to

I just tell them that I have never seen any proof that God exists. But they can believe in what ever they want too!

I tell them out-right that I am an Atheist, I make NO bones about it all, BUT they do get quite a Surprise when I also tell them and SHOW them that I have a Doctorate in Theology and Comparative Modern Religions AND still remain an Atheist.

i give them time to talk i just beleive what i think is true in my mind

I tell them that I am not a believer and that I have my own faith (in the secular meaning of this word)

i think my feelings r private to me how i feel should show by how i live

If the subject of religious beliefs comes up, I'll usually just say that I'm an agnostic and don't subscribe to any particular religious dogma. I'm spiritual in that I have an awe of Nature and a wonder about our Universe and the other "Big Questions," but unless somebody seems to want to discuss that topic further, I don't push it.

If they are not completely mindless and if that is where the conversation is headed I will tell them. If I think they'll listen I will further explain myself. This is kind of like coming out of the 'Atheist Closet'. Not as threatening as coming out in the traditional sense though.

I still avoid it, don't want to start fights.

If I'm at school they can usually tell from the FFRF and SSA stuff I have on my backpack and laptop. If we are talking about religion/politics I don't shy away from saying "Yeah no, I'm an Atheist so using the bible to justify X doesn't work for me".

It depends on who they are to me and what the situation is but I generally don't bring religion up. But if I'm asked, I'm honest and share my beliefs. I might also ask them what they believe about God. I've never gotten the same answer.

I announce at the earliest convenience that I am a card carrying, flag waving atheist. Then I explain that was a figure of speech. I have no card or flag.