I think I need not say it [mortification] is not the dissimulation of a sin. When a man on some outward respects forsakes the practice of any sin, men perhaps may look on him as a changed man. God knows that to his former iniquity he hath added cursed hypocrisy, and is got in a safer path to hell than he was in before. He hath got another heart than he had, that is more cunning; not a new heart, that is more holy.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 25