Mortification refers in Christian theology to the subjective experience of Sanctification, the objective work of God between justification and glorification. Literally it means the 'putting to death' of sin in a believer's life. (Colossians 3:5) Reformed theologian J.I. Packer describes it in the following way: "The Christian is committed to a lifelong fight against the world, the flesh and the devil. Mortification is his assault on the second."  Christians believe that this internal work against sin is empowered by the Holy Spirit and so therefore is also part of regeneration.
Historical Interpretations of Mortification
Roman Catholic theology frames mortification within the believer's personal struggle against sin. According to the Catholic Encyclopedia, "What it slays is the disease of the soul, and by slaying this it restores and invigorates the soul's true life."
Calvinism and Reformed theology
John Calvin observed that if believers died with Jesus then He destroys our sinful earthly members and their lust, "so that they may no longer perform their functions." Mortification in Reformed theology has been generally understood to be the subjective experience of sanctification.
- James Packer, God's Words (London: Christian Focus, 1998): 180.
- 'Mortification,' The Catholic Encyclopedia. (New York: Robert Appleton Company, 1911) http://www.newadvent.org/cathen/10578b.htm Retrieved 20 May 2009.
- John Calvin, Institutes of the Christian Religion (II.16.7).
- Bruce Demarest, The Cross and Salvation: The Doctrine of Salvation (Wheaton: Crossway Books, 1997): 402.