Why do some Christians often assume you're of the same faith?

Wow!! This is a good question and as I read through the answers, I agree with every one. Just keep it simple and try not to be in the position of having to be more frank in stating you're simply not of any religion persuasion and don't really wish to be. This sometimes leads to a "sermon-ette." Be kind to people at all levels of their ignorance.

I get that here a lot too, small west Texas town. I think it's mostly the assumption that everyone goes to church on Sunday, more than anything to do with you personally. Sorta like asking you what high school you went to. Football and church sorta rule the roost in Texas.

I just tell them I do not attend services and smile and leave it at that. If they want to push the issue, I either tell them it's a personal matter and shut my mouth again, or if I'm feeling arguementative I'll tell them I'm Buddhhist and let the sparks fly lol!

no its just theyre a very small pertange of athesit less then 1% in the US however there is 90% chritians.you should just respond with honesty like i dont go to church im athesit its simple it was a simple mistake and im sure if you tell them theyre feel sorry .

I don't know, but I think that they think you have t o be a Christian to be normal, or something. Maybe it is because there are so many of them, and they don't think any of the rest of us have valid beliefs. maybe it is because they think that anyone who is not Christian has to be of the devil, and they can't reconcile a normal looking person being of the devil. I think you have asked a very good question, and I have to agree that it is perplexing.

Just tell them you are an Atheist. Have some courage.

Tell them you have a personal belief that you are quite satisfied with, and to not judge you.

because thay think thay run the planet