Many Christians are somewhat irritated when atheists bring up Unicorns. Atheists do that mostly to show that in general, logic can’t prove a negative: the fact that atheists can’t disprove god’s existence is not proof of his existence; the way to show this to the believer is to ask them to disprove the existence of unicorns.

Because everyone knows that unicorns don’t exist.

Except that Isaiah 34:7 does mention unicorns.

Well, believers usually don’t know that fact either.