25 Thus the heavenly King arrayed Himself in armour of humility, and so conquered the bitter one, and drew from him a good answer as a sure pledge [of victory]. This is the armour concerning which Paul said, that by it we humble the loftiness that exalts itself against the knowledge of God. For Paul had received the proof of it in himself. For as he had been warring in pride, but was conquered in humility, so is to be conquered every lofty thing which exalts itself against this humility.
For Saul was journeying to subdue the disciples with hard words, but the Master of the disciples subdued him with a humble word. For when He to whom all things are possible manifested Himself to him, giving up all things else, He spoke to him in humility alone, that He might teach us that a soft tongue is more effectual than all things else against hard thoughts. For neither threats nor words of terror were heard by Paul, but weak words not able to avenge themselves: Saul, Saul, why do you persecute Me? But the words which were thought not even capable of avenging themselves, were found to be taking vengeance by drawing him away from the Jews and making him a goodly vessel.
He who was full of the bitter will of the Jews, was then filled with the sweet preaching of the cross. When he was filled with the bitterness of the crucifiers, in his bitterness he made havoc of the churches. But when he was filled with the sweetness of the Crucified, he embittered the synagogues of the crucifiers. Our Lord then strove with humble voice with him, who had been warring against His churches with hard bonds. Thus Saul, who had been binding the disciples with bitter chains, was bound with pleasant persuasions; that he might not again cast the disciples into bonds; since he was bound by the Crucified, Who puts to silence evil voices, whom all they that were set against Him could not bind or injure.
But when Paul ceased from binding the disciples, he himself was bound with chains by the persecutors. But when he was bound with chains, he loosed the bonds of idolatry by his bonds.
Source: On Our Lord (New Advent)