John Owen on mortification:
[To mortify means to kill,] and the end aimed at in this duty is destruction, as it is in all killing: the utter ruin, destruction and gradual annihilation of all the remainders of this cursed life of sin. Indwelling sin has been dethroned and dealt its death blow through the believer’s union with Christ in His death. Now, with the Spirit’s aid, the Christian must spend his lifetime draining sin’s lifeblood. We may not relax, for sin “will no otherwise die, but by being gradually and constantly weakened; spare it and it heals its wounds, and recovers strength.”
Mortification, which “consists in a constant taking part with grace… against the principle, acts and fruits of sin,” is often painful and unattractive; Christ compared it to plucking out an eye or cutting off a limb; but it is one aspect of the way of life that no Christian dare neglect.