Hence are those manifold cautions that are given us to take heed that we be not deceived, if we would take heed that we do not sin (see Eph. 5:6; 1 Cor. 6:9; 15:33; Gal. 6:7; Luke 21:8). From all which testimonies we may learn the influence that deceit has into sin, and consequently the advantage that the law of sin has to put forth its power by its deceitfulness. Where it prevails to deceive, it fails not to bring forth its fruit.
The ground of this efficacy of sin by deceit is taken from the faculty of the soul affected with it. Deceit properly affects the mind; it is the mind that is deceived. When sin attempts any other way of entrance into the soul, as by the affections, the mind, retaining its right and sovereignty, is able to give check and control unto it. But where the mind is tainted, the prevalency must be great; for the mind or understanding is the leading faculty of the soul, and what that fixes on, the will and affections rush after, being capable of no con sideration but what that presents unto them. Hence it is, that though the entanglement of the affections unto sin be oftentimes most troublesome, yet the deceit of the mind is always most dangerous, and that because of the place that it possesses in the soul as unto all its operations. Its office is to guide, direct, choose, and lead; and “if the light that is in us be darkness, how great is that darkness!” [Matt. 6:23].
Overcoming Sin & Temptation (Wheaton, IL; Crossway; 2006) p. 295.