Mortify means “to kill sin.” It is a simple way of saying, “Don’t let sin raise its ugly head.”
David said, “Thy word have I hid in my heart that I might not sin against you” (Ps. 119:11). It’s all about your thoughts. Proverbs says that as a man thinks in his heart, so he is (Prov. 23:7). If I expose myself to sin, temptation, and the garbage in the culture, I will have a hard time suppressing sin because I am activating all the beachheads of sin in my human flesh. So, I have to control my mind and have it renewed continually in the Word of God so that it is on the righteous path, not the sinful path. I am not talking about some kind of esoteric experience but daily exposure to the Word of God so that it is running around in your heart, soul, and mind all the time.
One reason to sing hymns is that they put theology in your heart in memorable ways. Very few minutes pass in my day where I am not singing a hymn in my head, which is a recitation of the things I believe—the righteous truths that insulate me from sin.
Killing sin is not something that happens at one point in time. Paul said, “I die daily” (1 Cor. 15:31). So, you are called to kill sin all the time, but in the strength of the Spirit and through the truth of Scripture.