I would tell that person that I am a Secular Humanist, a non-believer and give him my reasons for why I am. I would also tell that person that I have no interest in hearing anything they have to say, wish them a good day and walk away.

I have had that happen to me, more than once. Some, once they find that I’m an atheist, leave others want to engage me. Some think that because I don’t believe in the Christian god something must have happened in my life to be angry at god. To that I say, yes, something did happen in my life, but not a tragic event that made me angry at god. I respond with I read the KJ version and the American Standard versions of the Christian bible and applied basic logic and realized that this book was not true. Some get angry other just walk away. I don’t get angry myself I just roll with it and be honest.

I think it would depend upon the actual words used.

Without any other context than what is stated in the question above, I would immediately start asking questions to clarify their agenda. If they were trying to convert me it would be a much shorter chat than if they were honestly interested in my perspective and had a substantive perspective of their own to share.

I don't like strangers approaching me

How much time do I have on my hands for this right now and what are my other priorities?

In the where?

What's he/she trying to sell? Discussions on religion rarely remain detached.

He’s a missionary.

I'd happily oblige.

"god position"? I'd talk about the Kama Sutra!

I wanted be friendly back...

Pithy for this person. (Actually I would be snickering.)

Since I was never a believer, I would go back to the woman who took care of me during the day while my parents were working. She was one of the most religious person's I ever met.

I do not mind when people try this, but I usually turn the tables and start asking questions to them. And most of them can't answer and they usually leave frustrated and confused.

Suspicion.

I would be 'up front' about my atheism.

If they were reasonable I'd probably suggest they were going to get nowhere and bring the conversation to a close in as pleasent a manner as possible.

If they were and arse, I'd go 'full on confrontational atheist'.

If they happened to be interesting in some way I might engage in gentler debate to find out more about them.

If it's a stranger then I don't have a problem to discuss within decent manners. If the conversation moves towards interesting points then its a win win. If it goes south then its easy to wrap it up and it a day. That person is a stranger to me as I am a stranger to he/she anyway.....

TBH, I enter into any such conversations these days with a prejudicial attitude (so I just don't enter, LOL!). There usually is an agenda, or just a desire to argue.

I'd back up slowly.... maintaining 21+ feet separation... place my hand on my sidearm.

I don't do friendly chat involving god, church or religion. And that will go in the front of my mind.

I always seem to be happy with it as I have red the bible and want to know their take on some of the stories there. I find that most people who are religious have no clue about what the bible says. They have snippets but not what it actually says. They get very perplex and upset when one asks them.

It all sounds a bit creepy and weird.

Will my response anger that person?

This actually has happened to me.

I told them I would be happy to if we both only say things that can be proven, it was a short conversation.

In the back of my mind would be try to stay nice and open but what kind of shit are they going to throw out . My position would be to welcome an academic conversation but if they do not read then to hell with it.