Greek Expositor (anonymous) — as excerpted in the Catena Aurea on Luke 11:9-13

“(Severus Antioch.) Or by the word knock perhaps he means seeking effectually, for one knocks with the hand, but the hand is the sign of a good work. Or these three may be distinguished in another way. For it is the beginning of virtue to ask to know the way of truth. But the second step is to seek how we must go by that way. The third step is when a man has reached the virtue to knock at the door, that he may enter upon the wide field of knowledge. All these things a man acquires by prayer. Or to ask indeed is to pray, but to seek is by good works to do things becoming our prayers. And to knock is to continue in prayer without ceasing.”