“The second error has to do with the composed sense, namely, we should not claim that because the divine foreknowledge already exists beforehand, Peter is in reality not able not to sin, as if because of the preesisting divine knowledge he has lost something of his freedom and power not to sin in reality, should he so will. For I would not hesitate to call this sort of interpretation an error from the point of view of the faith.
Indeed, even though that knowledge did exist beforehand, it was just as truly within his power not to sin as it would have been had that knowledge not existed, and he was just as truly able to refrain from the act in light of which he was foreknown to be a future sinner as he would have been had that knowledge not existed, as has been explained; thus this interpretation is not the one that the theologians have in mind.
Rather, they are claiming, absolutely correctly, that given the divine foreknowledge, Peter is not able in the composed sense not to sin, because these two things, namely, Peter’s being such that he is not going to sin, and God’s knowing that he is going to sin, cannot both obtain together. But if, as is now truly possible, he were not going to sin, then that knowledge would not have existed in God, and so that knowledge, which would not have existed if, as is possible, Peter were not going to sin, does not in any way prevent Peter’s now being able in the divided sense not to sin, in just the way he would have been able not to sin had such knowledge not existed beforehand. (Concorida. Disputation 52. P186. Translated by Fredosso.)