36 For even though our Lord in His grace had beforetime brought the majesty of His Godhead into humility, yet afterwards in His justice He willed not again to bring back to humiliation the littleness of manhood which had been made great. But because it was necessary that the persecuting disciple should learn endurance of persecution, while yet it was impossible that the Master should again come down and be persecuted afresh; He taught him by voice that which could not be taught by deeds. Saul, why do you persecute Me? The explanation of which utterance is this—“Saul, why are you not persecuted in Me?”
But in order that Saul might not suppose that it was because of His weakness our Lord was persecuted, the strength of the overpowering light which shone upon him, convinced him. For if the eyes of Saul could not endure the shining of that light, how could the hands of Saul bind and fetter the disciples of the Lord of that light? But his hands had fettered the disciples, that he might learn their power in their bonds; while his eyes could not endure the beams, that by their strength he might learn his own weakness.
But had not the power of that light shone upon him, when the Lord said to him; Saul, why do you persecute Me? Then because of the madness of the pride wherein Paul was set up at that time, he would perhaps have said this to Him, “I am persecuting You for this reason, because You have said, Why do you persecute Me? For who is there that would not persecute You, when You, with such strength, troublest Your persecutor with these feeble cries.” But the humility of our Lord was heard in the voice, and the power of the light shone forth in the beams. So Paul could not despise the humility of the voice, because of the glory of the light.
Source: On Our Lord (New Advent)