Every time I discuss the issue of religion with any non-Objectivist, I usually cover all the bases I understand best: identity, causality, the "super" natural and mans means of knowledge – revelations v. reason. However, my favorite ground for disqualification of the notion of a god lies in the nature of miracles.
Most people confuse miracles with magic tricks. They confuse illusory tricks with metaphysical impossibility. Could magicians really get a normal rose to speak? Could Jesus really break a normal piece of bread and break into a thousand parts? No, the discussion here is not about whether they have found a special rose that could speak or special bread that would break itself into a thousand different pieces. If there was really something special about the thing, then why would such an act even qualify as a miracle? It doesn't. And it doesn't because there is nothing "miraculous" about things acting the way they are supposed to act. In essence, a miracle has to this: It has to make a thing act in violation of its properties, its identity, its nature. In technical terms, it has to violate causality. It has to make a thing act against its nature – it would require that a normal rose speak instead of blooming and that a regular piece of bread break into a thousand parts.
As Greg Perkins puts it in his great essay, "In short, a genuine miracle requires a thing to act against its own identity—to have a contradictory identity—to literally not be what it is, which is incoherent."
As far as the magicians go, any successful magician will tell you that magicians have to closely conform to the identity of things to perform a trick. If he expects a heavy rock to fly in spite of gravity expecting a miracle in the course of a trick, he will without doubt end up making an ass of himself.
Update: A few corrections.