Reflecting on why God put Abraham’s faith to the test by commanding him to offer his son, Lewis says,

“If God then is omniscient, he must have known what Abraham would do, without any experiment. Why then this needless torture?” But as St. Augustine points out, whatever God knew, Abraham at any rate did not know that his obedience would endure such a command until the event taught him: and the obedience which he did not know that he would choose, he cannot be said to have chosen. The reality of Abraham’s obedience was the act itself; and what God knew in knowing that Abraham “would obey” was Abraham’s actual obedience on that mountain top a that moment. To say that God “need not have tried the experiment” is to say that because God knows, the thing known by God need not to exist. (The Problem of Pain, 101)