Phil Johnson via Reformed Bibliophile:
Mortify your evil thoughts…“Mortify, therefore, your members which are upon the earth…fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry,”…this is a recurring theme in the Apostle Paul’s writings, this idea of putting to death sin in your body, mortifying the sin. Romans 8:13, “If you live after the flesh, you shall die; but if you, through the Spirit, do mortify the deeds of the body, you shall live.” What he’s saying is this: put your evil thoughts to death. Deal with them ruthlessly. Don’t allow them any breathing room. Choke the life out of them. Mortify them. Romans 13:14, “Put ye on the Lord Jesus Christ and make not provision for the flesh, to fulfill the lusts thereof.”
…Carry it through to the end, because sinful thoughts are a fierce, deadly enemy that must be met with lethal force, choked out of existence, rooted up, exterminated, and utterly purged from our lives. That’s the only way to deal with sin in your life—any sin. Understand what Jesus was saying when He said, “All that defiles you comes from your heart,” He’s saying that every sin that troubles you is hatched in your mind! If you can control your mind, if you can mortify that sin that’s in your mind, that is the pathway to sanctification. If you don’t do that, every wicked thought will destroy you, if you don’t destroy it. And you’ll never get control of your thought life if you’re not proactive, deliberate, ruthless in mortifying and putting to death the sin that’s in your heart. . .