1.) Before it was created, the universe was possible.
2.) The possibility of the universe was either intrinsic to the Creator or not.
3a.) If intrinsic, then the uncreated (i.e. some intrinsic divine feature) is created.
3b.) If not intrinsic, then a created thing (i.e. something other than the Creator) is not created.
Both are impossible, and therefore before the universe was created it was impossible, which is a contradiction.
A thing is possible before it exists only so far as we assume a previous time, since where there is no later time for X to exist X is not possible, just as it is impossible to win a race after it is over. And so (1) would only be an appropriate description of the universe if the universe arose from some previous time, which no account of creation assumes and which is contrary to the idea of creation.
Nevertheless, this more begins a discussion than ends it. If time and possibility are logically implicated in this way, then how do we avoid saying that God did something impossible in creating time?
One response would be that the only universe that could be created – in fact, the only sort of universe that would be possible – is one with no first time. Take that, Kalam.
(I never liked that proof anyway.)