Another Confused Atheist

In his desperate attempt to restore Richard Dawkins' idiotic The God Delusion to some level of credibility, Steven Weinberg makes an exceedingly odd statement about Anselm's ontological proof of the existence of God:

From the monk Gaunilo in Anselm’s time to philosophers in our own such as J. L. Mackie and Alvin Plantinga, there is general agreement that Anselm’s proof is flawed, though they disagree about what the flaw is. My own view is that the proof is circular: it is not true that one can conceive of something than which nothing greater can be conceived unless one first assumes the existence of God.

While technically accurate in his summary of the criticism of the ontological argument, Steven Weinberg neglects to mention that Plantinga has reformulated the ontological proof in a logically near-unassailable fashion (it relies on axiom S5, which I understand has some few dissenters). But more interesting is his reason for dismissing it. Weinberg states that one must first assume God before one can conceive God. It is almost needless to point out that this makes no sense. You can't conceive something until you believe it exists? So, presumably, I can't conceive of a centaur without believing it exists? But let's be generous, and presume Weinberg doesn't mean what he appears to say. How might we understand his statement in a manner that gives some sensible meaning to it? It seems likely that Weinberg understands the existence of God to somehow allow for the "nothing greater than" clause to be coherent. If this is the case, Weinberg is implying that a metaphysical ordering of goods cannot exist without God. The irony is that atheists have long fought against the notion that objective ethics are an impossibility in atheism- but in order to hold the ontological argument at bay, Weinberg is willing to sacrifice such a possibility.