Listen to John Owen on the consequences of not mortifying (basically, killing) sin daily in our lives. Classic!
“Let him pretend what he will, he has slight thoughts of sin; at least, of sins of daily infirmity. The root of an unmortified course is the digestion of sin without bitterness in the heart. When a man has confirmed his imagination to such an apprehension of grace and mercy as to be able, without bitterness, to swallow and digest daily sins, that man is at the very brink of turning the grace of God into lasciviousness and being hardened by the deceitfulness of sin. Neither is there a greater evidence of a false and rotten heart in the world than to drive such a trade. To use the blood of Christ, which is given to cleanse us (1 John 1:7; Titus 2:14); the exaltation of Christ, which is to give us repentance (Acts 5:31); the doctrine of grace, which teaches us to deny all ungodliness (Titus 2:11-12), to countenance sin is a rebellion that in the issue will break the bones. At this door have gone out from us most of the professors that have apostatized in the days wherein we live. For a while most of them were under convictions; these kept them unto duties, and brought them to profession; so they “escaped the pollutions that are in the world, through the knowledge of our Lord Jesus Christ” (2 Pet. 2:20): but having got an acquaintance with the doctrine of the gospel, and being weary of duty, for which they had no principle, they began to countenance themselves in manifold neglects from the doctrine of grace. Now, when once this evil had laid hold of them, they speedily tumbled into perdition.”
John Owen, Of the Mortification of Sin in Believers
