35 And if you should say; “What necessity was there that He should speak humbly? Could He not have convinced him also through the greatness of the light?” Know, you that questionest, that this rejoinder may be returned to you; that because it was necessary that He should speak humbly, He therefore spoke humbly. For by Him Who is wise in all things, there was done there nothing that was not meet to be done. For He Who has given knowledge to artificers to do each thing severally with the instrument meet for it, does He not Himself know that which He gives others the power of knowing?
Therefore whatsoever has been wrought or is being wrought by the Godhead, that very thing that is wrought by Him at that time, is for the furtherance of [God's] working at that time, even though to the blind the Divine orderings seem contrariwise. But that we may not restrain by constraint of words a wise enquirer, one that wishes to grow by true persuasion as the seed by the rain-drops; know, O enquirer, that because Saul was a persecutor, but our Lord was endeavouring to make him persecuted instead of persecutor, therefore He of His wisdom made haste to cry— Saul, why do you persecute Me?— in order that, when Saul who was being made a disciple, heard Him Who was making him a disciple, saying, Why do you persecute Me? he might know that the Master Whose servant he was becoming, was a persecuted Master, and so might quickly cast away the persecution of his former masters, and might clothe himself in the persecuted state of his persecuted Master.
Now any master who wishes to teach a man anything, teaches him either by deeds or by words. But if he teach him neither by words nor by deeds, the man cannot be instructed in his craft. So that, even though our Lord did not teach Paul humility by deeds, yet by voice He taught him endurance of persecution which He could not teach him by deed. For before our Lord was crucified, He taught His disciples humble endurance of persecution by deed. But after He had finished His persecution by crucifixion, as He said, Lo! All things are finished. He could not vainly return and begin again anything which once for all had been wisely finished. Or why again do you seek for the crucifixion and shame of the Son of God?
Source: On Our Lord (New Advent)