Principle number one, then, is: Know, rest in, think through, and act upon your new identity — you are in Christ.
Second, Paul goes on to expose the workings of sin in every area of our lives (Col. 3:5–11). If we are to deal with sin biblically, we must not make the mistake of thinking that we can limit our attack to only one area of failure in our lives. All sin must be dealt with.
Third, Paul’s exposition provides us with practical guidance for mortifying sin.
1. Learn to admit sin for what it really is. Call a spade a spade — call it “sexual immorality,” not “I’m being tempted a little”; call it “impurity,” not “I’m struggling with my thought life”; call it “evil desire, which is idolatry,” not “I think I need to order my priorities a bit better.”
2. See sin for what your sin really is in God’s presence. “On account of these the wrath of God is coming” (3:6). The masters of the spiritual life spoke of dragging our lusts (kicking and screaming, though they be) to the cross, to a wrath-bearing Christ. My sin leads to — not lasting pleasure — but holy divine displeasure.
3. Recognize the inconsistency of your sin. You put off the “old man,” and have put on the “new man” (3:9–10). You are no longer the “old man.” The identity you had “in Adam” is gone. The old man was “crucified with him [Christ] in order that the body of sin [probably “life in the body dominated by sin”] might be brought to nothing, so that we would no longer be enslaved to sin” (Rom. 6:6). New men live new lives. Anything less than this is a contradiction of who I am “in Christ.”
4. Put sin to death (Col. 3:5). It is as “simple” as that. Refuse it, starve it, and reject it. You cannot “mortify” sin without the pain of the kill. There is no other way!
But notice that Paul sets this in a very important, broader context. The negative task of putting sin to death will not be accomplished in isolation from the positive call of the Gospel to “put on” the Lord Jesus Christ (Rom. 13:14). Paul spells this out in Colossians 3:12–17. Sweeping the house clean simply leaves us open to a further invasion of sin. But when we understand the “glorious exchange” principle of the Gospel of grace, then we will begin to make some real advance in holiness. As sinful desires and habits are not only rejected, but exchanged for Christ-like graces (3:12) and actions (3:13); as we are clothed in Christ’s character and His graces are held together by love (v. 14), not only in our private life but also in the church fellowship (vv. 12–16), Christ’s name and glory are manifested and exalted in and among us (3:17).
Source: The Practice of Mortification by Sinclair Ferguson | http://www.ligonier.org/learn/articles/practice-mortification/
Andrew Groves 