If I'm lucky, I will be asked this question glibly. Typically though, it's phrased as an assertion - "Yeah, OK, says the guy who believes in fairytales."
In the first place, this is really a veiled Appeal to the Stone fallacy:
This argument, of course, fails. Hearing it tells me immediately the speaker has at best an equally shaky justification for their beliefs as I do mine.
Problem 1: Once we dispose of the emotional appeal that is equating a proposition, asserted to be true, to a story fabricated for uncritical children, "fairytale" reduces to mean "fiction". That in mind, the objection, "You believe a fiction" is literally not an argument. It's a proposition, which is all well and good but a proposition isn't true by virtue of being made. It is a starting point after which justification is given to prove it.
Problem 2: If someone believes in a fiction, so what? I don't mean to say pragmatically, "self delusion isn't good but it's harmless so whatever" - I mean, why ought I believe self delusion is bad at all? If it's bad, you'll need to justify why that's the case, not just assume your position. If you can just assume shit without a rationale, then I can equally assume that the "fairytale" is real after all. If that's not OK, I agree! Neither is assuming belief in a fairytale is bad.
Problem 3: Even if I grant self delusion is bad, something being fictional doesn't tell you anything about anything else the believer believes, says or does unless you presuppose a system of deduction. OK, person A believes a fiction. Ergo quid? Maybe he's wrong about that, but right about literally everything else. Unless you've already assumed a worldview, nothing necessarily follows from that. And again, if you can assume your worldview, then I can assume mine.