A metaphysical principle entailing theism?

Take the following metaphysical principle, connecting logic, knowledge and truth: 'If it is logically impossible to know that p, then p is necessarily false'. This principle seems to be cogent. For, if a given proposition p could be true, then, plausibly, there is a possible world in which some subject knows that p is true. In other words, if in all possible worlds all subjects do not know that some proposition is true, then, plausibly, that is because that very proposition cannot in fact be true.

Well, on a cartesian view of knowledge, that is, to know p is to be certain that p is true, the above principle has an interesting consequence. For, take for p the proposition 'God does not exist'. It seems reasonable to hold that it is impossible to know that God does not exist. For, whatever the arguments against God, there will always be some (perhaps an extremely remote) possibility that God does exist after all, so that we can never truly say, on the cartesian view, that we know that God does not exist. But then it follows that it is necessarily false that God does not exist. Hence, it is necessarily true that God exists.

One might object that it is also impossible to know that God exists. And thus, by similar reasoning, it would follow as well that it is necessarily true that God does not exist. However, I would argue that there is a possible world in which some subject can truly say that he or she knows that God exists. Take a possible word in which God exists and in which there is an afterlife, such that all who enter the afterlife in that world will encounter the divine. In that case, those subjects who enter the afterlife will in fact know that God exists. So, it is not impossible to know that God exists.

Note that a similar move to reject the argument for theism is not open to the atheist. For, if God does not exist, then, plausibly, there is no afterlife. And besides, even if there would be an afterlife, then entering it would not bring a subject in the epistemic condition of knowing that God does not exist.