There's different degrees of belief and sets of belief. Some beliefs are so far removed from reality that you aren't going to have much success arguing with them. If someone really believes that all atheists are evil, you probably aren't going to get very far with that person.

However, as a general rule for internet conversations, you will almost never convince the person you are arguing with that their position is wrong. However, if you are firm and polite and have better arguments on your side, you do a good chance of helping convince more moderate people who aren't part of the argument but are reading it.