OK---Prove it!

Don't bother.
When people make their unsubstantiated and ridiculous assertions just tell them you don't believe them and ask for some evidence.
When they claim faith point out that all religions claim faith but they contradict each other. Faith is not a reliable path to truth.

Im curious what drives your urge to say it? I don't mean to disagree, just curious what's behind it.

When asked, I simply respond with, "I am atheist." Sometimes the subject will come up in a peripheral sense, such as a gathering that traditionally includes prayer. Since I decline to participate in prayer, some inevitably notice. I don't exactly wear my disbelief on my sleeve, nor do I make any effort to hide or downplay it. I'm accustomed to having it come up frequently because I am the only atheist (as far as I know) in my extended family. I have a niece, a brother-in-law, and a son-in-law who are ordained ministers.

Every situation is asking for a proper reaction. If asked I will answer, but if they don't ask they will have to figure out from the conversation. Most of my friends are religious and I am not hiding being an atheist. I like to be with intelligent people where any opinion or believe can be challenged in a civilized way. At one occasion I had a conversation with a Rabbi. I told him that I am an Atheist. His wife overheard the conversation and she told me, you can't be an atheist you have a Jewish soul. I didn't challenge her. I have no idea what a Jewish soul is.

I'm pretty upfront about my atheism. It may be off putting to some but I expect my lack of religion to be respected the same and they expect their religiosity to be respected.

No need to tell them until they ask. and If they ask then one must try to tell them in more moderate way with smiley face. Like if someone ask why you don't go to Church then answer is simple, sorry dude i am not Christian, I am atheist, if you like Church then it is Ok, your believe has nothing to do with our friendship. If he still remain friend then be friend, if it annoys him then he deserve to leave him alone

If the subject comes up--surprising how seldom that is, given where I live--I just say I'm not a believer. Conversations usually move one to more interesting topics.

I personally am somewhat of a believer. As in I believe in a god but not so much in religion.

Faith in God yes not holy book religions. Some evolution God played a role in. Souls spirit are from God given to humans. Use logical reasoning in positive ways focus forward. Pro choice and pro life both. Science and God. All starter knowledge and skills is from evolution and God. All our natural given abilities can be used for good.

If I'm speaking to a religious fanatic at my front door who tries to put me down for my non-religious thinking, I usually tell them to stick their stupid religion where the sun never shines and go on down the street. To all others I simply state that I am not religious.

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.

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.

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".

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

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

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

I CANT and it is smart of me to not to

I never tell them

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.

Inspired them through the ideas put forward in the discussions

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

I slam their religion without mercy and give examples of why their respective religion is crap.

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.

Did tell my sisterthat, if I'm going to christian hell, i'll have plenty of Christian company. That usually ends her judgement.