Don’t. Unless the attack also involves a larger issue that needs to be discussed, don’t bother. The reason that I do not generally respond to personal attacks is that anybody who would believe it would be someone for whom I have no respect and for whose opinion (about me or anything else) I could not possibly care less about. Actually, to indulge in personal attacks is to admit weakness. For instance, I have seen people write foaming diatribes against William Lane Craig when, in a face-to-face encounter, Craig would soon have them whimpering like a whipped dog. The Internet is full of gutless wonders who hide behind a pseudonym and blithely engage in personal attacks while they lack both the intelligence and the integrity to engage in a respectful, straightforward debate. Don’t try to be reasonable with such characters. This would contravene the wise biblical injunction about putting pearls before swine. The Internet is a great tool. It allows you to have fascinating discussions with people from many backgrounds and perspectives. Unfortunately, it also means that you will encounter a fair number of idiots and assholes. Respond to the ones who want to have an intelligent discussion and let the others stew in their own juices. Nothing you can say will give intelligence to the idiot or remake the asshole into a decent human being. Ignore them.