In this we see the deceitfulness of sin. It gradually prevails to harden man's heart to his ruin (Heb. 3:13). Sin's expression is modest in the beginning but, once it has gained a foothold, it continues to take further ground and presses on to greater heights. This advance of sin keeps the soul from seeing that it is drifting from God. The soul becomes indifferent to the seed of sin as it continues to grow. This growth has no boundaries but utter denial of God and opposition to Him. Sin precedes higher by degrees; it hardens the heart as it advances. This enables the deceitfulness of sin to drive the soul deeper and deeper into sin. Nothing can prevent this but mortification. Mortification withers the root and strikes at the head of sin every hour. The best saints in the world are in danger of a fall if found negligent in this important duty!
John Owen - "The Mortification of Sin"