A reader (!) asks, "If we accept that man's cooperation with sufficient grace always remains in potency if he is not given an efficacious grace how, then, can we understand man's culpability in resisting sufficient grace when no efficacious grace to cooperate with it is granted?"
Hmmm, good question, and certainly a classic! What sort of guilt (culpa) is there in those who could not cooperate with grace?
Take first the example of the unbaptized baby. Until regenerated, the infant bears the culpa of original sin, and is subject to that sin's poena, punishment. Yet the child has no personal sins, no personal sins to be punished. (Hence Limbo, with its perfect natural happiness apart from supernatural beatitude. Here we have culpability without positive punishment, it seems to me.)
Second, what about the sinner, the person whose deeds have been evil, who does not receive the efficacious grace to cooperate with God? I'm not sure it's precisely the lack of cooperation that is culpable. (I'll have to check and see how the Church treats this!) I'll guess, for the moment, that our best course is to think about people who don't receive the actual efficacious grace to cooperate in the same way that we think of divine reprobation: God withholds supernatural assistance from them as a punishment for actual sins. It's their (our) malice, uncharity, and sloth that cause guilt and justify punishment -- not the mere lack of cooperation, and still less the lack of operative grace.
This has got to be one of the murkiest parts of theology. It would be easier if semi-Pelagianism were true, and there were no predestination. All this requires more thought.