My beliefs (religious and otherwise) are fundamental to who I am -- they define my moral code, and I don't want people to have a mistaken impression about that. If people are comfortable ascribing beliefs to me that aren't mine, I'm comfortable correcting it with a sentence or two.
Even "No, actually, I don't believe that, but let's not get into an unpleasant discussion" is better for me than allowing someone to go on believing that I think the world is flat.
I can understand that. I love that last line. I use that a lot.
But, may I ask who these people are? If they are friends who make an incorrect statement in passing conversation? Or are they total strangers or people you don't really socialize with who make this statement as a judgement?
I guess it depends on the people your dealing with and the circumstances that brought up the conversation. If it was a friend or someone you know who made a statement and was mistaken, I can understand the desire to correct it, and share you belief with them in a civil and polite way.
I personally don't waste my energy on people who make ignorant and uneducated comments, especially people who aren't involved in my life. It is not going to change thier mind and I am secure enough in my belief to know the truth so what do I care if they think the world is flat? It won't be my problem when they fall off the edge.
In my experiance, I have found that correcting them, no matter how politley, just seems to invite conflict. But, I don't imagine that you are the type to associate with ignorant people either ZipTheWonder.