12 But do what you are able; what the Apostle himself says in another place, which I had already begun to repeat; “Let not sin reign in your mortal body, to obey the desires thereof.” Lo, what I would not; evil desires arise; but obey them not. Arm yourself, assume the weapons of war. The precepts of God are your arms. If you listen to me as you should, you are armed even by that which I am speaking. “'Let not sin,' he says, 'reign in your mortal body.' For as long as you bear a mortal body, sin does fight against you; but let it not reign.”
What is, “Let it not reign”? That is, “to obey the desires thereof.” If you begin to obey, it reigns. And what is it to obey, but to “yield your members as instruments of iniquity unto sin”? Nothing more excellent than this teacher. What would you that I should yet explain to you? Do what you have heard. Yield not your members instruments of iniquity unto sin. God has given you power by His Spirit to restrain your members. Lust rises up, restrain your members; what can it do now that it has risen?
Restrain your members; yield not your members instruments of iniquity unto sin; arm not your adversary against yourself. Restrain your feet, that they go not after unlawful things. Lust has risen up, restrain your members; restrain your hands from all wickedness; restrain the eyes, that they wander not astray; restrain the ears, that they hear not the words of lust with pleasure; restrain the whole body, restrain the sides, restrain its highest and lowest parts. What can lust do? How to rise up, it knows. How to conquer, it knows not. By rising up constantly without effect, it learns not even to rise.
Source: Sermons on Selected Lessons of the New Testament (New Advent)