“For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live.” Romans 8:13 NKJV.
Mortify means ‘to put to death’ and betokens the act of God on the believer through the death of Christ and it is an act of the believer himself, as being responsible to God’s act, to put to death “the deeds of the body”.
“Why Mortification Is the Work of the Spirit
It is then, the work of the Spirit. For He is promised of God to be given unto us for this work. The taking away of the stony heart-that is, the stubborn, proud, rebellious, unbelieving heart-is in general the work of mortification that we treat of. Now this is still promised to be done by the Spirit, “I will give my Spirit, and take away the stony heart” (Ezek. 11:19; 36:26), and by the Spirit of God is this work wrought when all means fail (Isa. 57:17-18).
We have all our mortification from the gift of Christ, and all the gifts of Christ are communicated to us and given to us by the Spirit of Christ: “Without Christ we can do nothing” (John 15:15). All communications of supplies and relief, in the beginnings, increasings, actings of any grace whatsoever, from Him, are by the Spirit, by whom He alone works in and upon believers. From Him we have our mortification: “He is exalted and made a Prince and a Savior, to give repentance unto us” (Acts 15:31); and of our repentance our mortification is no small portion. How does He do it? Having “received…the promise of the Holy Ghost,” He sends Him abroad for that end (Acts 2:33). You know the manifold promises He made of sending the Spirit, as Tertullian speaks, “Vicarium navare operam,” to do the works that He had to accomplish in us.
The resolution of one or two questions will now lead me nearer to what I principally intend.
How the Spirit Mortifies Sin
The first [question] is: How does the Spirit mortify sin? I answer, in general, three ways.
By causing our hearts to abound in grace and the fruits that are contrary to the flesh, and the fruits thereof and principles of them. So the apostle opposes the fruits of the flesh and of the Spirit: “The fruits of the flesh”, says he, “are so and so” (Gal. 5:19-21); “but,” says he, “the fruits of the Spirit are quite contrary, quite of another sort” (vs. 22-23). Yea; but what if these are in us and do abound, may not the other abound also? No, says he, “They that are Christ’s have crucified the flesh with the affections and lusts” (v. 24). But how? Why, “by living in the Spirit and walking after the Spirit” (V. 25)-that is, by the abounding work of these graces of the Spirit in us and walking according to them. For, says the apostle, “these area contrary one to another” (v. 17); so that they cannot both be in the same subject in any intense or high degree. This “renewing of us by the Holy Ghost,” as it is called (Titus 3:5), is one great way of mortification; He causes us to grow, thrive, flourish, and abound in those graces which are contrary, opposite, and destructive to all the fruits of the flesh, and to the quiet or thriving of indwelling sin itself.
By a real physical efficiency on the root and habit of sin, for the weakening, destroying, and taking it away. Hence He is called a ‘spirit of judgment and…burning” (Isa. 4:4), really and consuming and destroying our lusts. He takes away the stony heart by an almighty efficiency; for as He begins the work as to its kind, so He carries it on as to its degrees. He is the fire which burns up every root of lust.
He brings the cross of Christ into the heart of a sinner by faith, and gives us communion with Christ in His death and fellowship of His sufferings: of the manner whereof more afterward.
-John Owen (1616-1683) From Overcoming Sin and Temptation by Kelly Kapic and Justin Taylor Published by Crossway Books Wheaton, IL 2006 Pages 60-61.