Thomas Aquinas

But if there were no evils in the world, much good would be lost to man, as well in respect of knowledge, as also in respect of desire and love of good: for good is better known in contrast with evil; and while evil results come about, we more ardently desire good results: as sick men best know what a blessing health is.

The Lord reveals to the prophets all things that are necessary for the instruction of the faithful; yet not all to every one, but some to one, and some to another.
And although God revealed in general to the prophets what He was one day to do regarding the salvation of the human race, still the apostles knew some particulars of the same, which the prophets did not know. Thus we read (Ephesians 3:4-5): "As you reading, may understand my knowledge in the mystery of Christ, which in other generations was not known to the sons of men, as it is now revealed to His holy apostles." Among the prophets also, the later ones knew what the former did not know; according to Psalm 118:100: "I have had understanding above ancients," and Gregory says: "The knowledge of Divine things increased as time went on" (Hom. xvi in Ezech.).

And thus even the good movement of the free-will, whereby anyone is prepared for receiving the gift of grace is an act of the free-will moved by God. And thus man is said to prepare himself, according to Proverbs 16:1: "It is the part of man to prepare the soul"; yet it is principally from God, Who moves the free-will. Hence it is said that man's will is prepared by God, and that man's steps are guided by God.