3 Wherefore, see how the Lord in this passage exhorted His disciples to prayer, when He said, “You could not cast out this devil because of your unbelief.” For then exhorting them to prayer He ended thus; “this kind is not cast out but by prayer and fasting.” If a man must pray, to cast out devils from another, how much more to cast out his own covetousness? How much more to cast out his own drunkenness? How much more to cast out his own luxuriousness? How much more to cast out his own uncleanness?
How many things in a man are there, which if they are persevered in, allow of no admission into the kingdom of heaven! Consider, Brethren, how a physician is entreated for the preservation of temporal health, how, if any one is desperately ill, is he ashamed or slow to throw himself at a man's feet? To bathe in tears the footsteps of any very able chief physician? And what if the physician say to him, “You can not else be cured, except I bind you, and use the fire and knife”? He will answer, Do what you will, only cure me.
With what eagerness does he long for the health of a few days, fleeting as a vapour, that for it he is content to be bound, and submit to the fire, and knife, and to be watched, that he neither eat nor drink what, or when, he pleases! All this he will endure, that he may die a little later; and yet he will not endure ever so little, that he may never die. If God, who is the Heavenly Physician over us, says to you, “Will you be cured?” what would you say but “Yes.” Or it may be you would not say so, because you fancy yourself to be in health, that is, because you are more grievously sick.
Source: Sermons on Selected Lessons of the New Testament (New Advent)