[benguino]

I'm going to try to focus back on the OP's original intent for this thread.

So, one may argue that many religions, as a whole, keep society from progressing. I would even say myself that that is probably true, but I digress.

The point of the matter however, was that the OP was asking about the ethics/morality of an atheist ridiculing a theist in a way that the OP described. Now this is highly dependent on the situation, but in general, I would find that behavior non-respectable (however, in certain circumstances I would go as far to say that the theist "had it coming"). However, if the situation is as follows:
Atheist: "So what's your religion."
Theist: "Oh, I'm _____. Why did you ask?"
Atheist: "LOL, YOU'RE ****ING RETARDED!!!!1"
then clearly the atheist is the one preventing society to progress by acting immaturely and lowering the theist's self esteem (making them less likely to accomplish something that could be worth while to society).

Although religion, overall as a whole, may be keeping society from progressing, you can't assume that that is the true of each and every single individual.