[LXXX. Ben.]
On the words of the Gospel, Matthew 17:19 , “Why could not we cast it out”? Etc., and on prayer.
1. Our Lord Jesus Christ reproved unbelief even in His own disciples, as we heard just now when the Gospel was being read. For when they had said, “Why could not we cast him out?” He answered, “Because of your unbelief.” If the Apostles were unbelievers, who is a believer? What must the lambs do, if the rams totter? Yet the mercy of the Lord did not disdain them in their unbelief; but reproved, nourished, perfected, crowned them. For they themselves, as mindful of their own weakness, said to Him, as we read in a certain place in the Gospel, “Lord, increase our faith. Lord,” say they, “increase our faith.” The knowing that they had a deficiency, was the first advantage; a greater happiness still, to know who it was of whom they were asking. “Lord,” say they, “increase our faith.” See, if they did not bring their hearts as it were to the fountain, and knocked that that might be opened to them, out of which they might fill them. For He would that men should knock at Him, not that He might repel those that knock, but that He might exercise those who long.
Source: Sermons on Selected Lessons of the New Testament (New Advent)