The only truly rational position is that of the agnostic, who maintains that the existence—or non-existence—of a supreme being is not only unknown, but probably unknowable.

I think this needs qualifying. There are three distinct kinds of "agnostic", two of which are entirely honourable and the other which is highly disreputable and cowardly. First, you've heard of religion, you were taught it, you don't bother with it except for weddings and you don't really give a damn. Entirely sensible and honourable. Second, you have given the matter much thought, taking all the evidence into account, and are highly critical of the concept of a God who breaks every known law of nature and who is infinitely more inexplicable than all the things he's supposed to be there to explain. You conclude that the chances of his existence are so small that you might as well get on with your life as if he isn't there, even though you can't (and don't want to) actually prove it. Highly honourable and sensible. Third, you assert that you don't know either way, he could exist but he might not and you're happy with that (after all, it chucks in a bit of insurance to boot). You haven't been anything like critical enough of the supposed evidence and, frankly, all that religious background of yours has made you feel a bit scared of saying he doesn't exist. You're a fence-sitter. Unnecessary and disreputable. Get off your knees!

Of course, the second category comprises those we call atheists. Far from "not believing in God", or stating with certainty that he doesn't exist, we are simply waiting for the evidence that we are pretty confident will not come along (a blazing chariot of fire from the heavens would do it for me, though your mileage may vary). We merely shrug in believers' general direction.