"The deceitfulness of sin"! In this context, to be deceitful is to be seductively and enticingly misleading. Sin promises what it cannot deliver. It promises pleasure, contentment, fulfillment—life—but its delivery on these things is fleeting and ultimately unsatisfying. Its deceitfulness is the very reason why it has addictive qualities. It lures us on to try to capture what it can never deliver.
The pleasure is never quite enough to produce the contentment and fulfillment one desires. Thus, people are forced into greater and deeper perversions until it results in death. All along the way, from its inception to death, sin quietly produces hardness of heart. Like a callus that forms over a break in a bone or stiffens a person's joints, sin paralyzes right action.
"Hardness" is translated from skleruno, from which name for the disease multiple sclerosis is derived. In a moral context, it means "impenetrable," "insensitive," "blind," "unteachable." A hardened attitude is not a sudden aberration, but the product of a habitual state of mind that reveals itself in inflexibility of thinking and insensitivity of conscience. Eventually, it makes repentance impossible. The will to do right is completely gone.
The will is the power or faculty by which the mind makes choices and acts to carry them out. An old adage says: "Sow an act and reap a habit; sow a habit and reap a character; sow a character and reap a destiny." At first, against his will, a person engages in some forbidden pleasure out of weakness, curiosity, or sheer carnality. If the practice continues, he sins because he cannot help doing so; he is becoming addicted to it. Once a sin becomes a habit, he considers it to be almost a necessity. When it becomes a necessity, the destiny is produced.