The life of evil men consists of nothing but "deceiving and being deceived" (2 Tim. 3:13). This is also the character of our enemy. He is deceitful. Against such a one no security exists but vigilance. (73, emphasis mine)

The basis for the efficacy of deceit is its effect upon the mind. For sin deceives the mind. When sin attempts to enter into the soul by some other way (such as by the affections), the mind checks and controls it. But when deceit influences the mind, the chance of sinning multiplies. (74)

So sin aims first of all to distract and to divert the mind from the discharge of its duty...The duty of the mind consists of two things. God requires these of us in our obedience to Him. The first is to keep the mind in such a posture and framework that it is obedient and watchful against all sinful enticements. The second is to attend to and perform all particular actions as God requires according to His will...Indwelling sin tries to divert and to draw away the believer from doing these things. (77)

"Be clothed with humility" says the apostle (1 Peter 1:17). This is what becomes us as the only safe disposition...How is this humble disposition obtained? How is it kept? It is only achieved by a constant, deep apprehension of the evil, vileness and danger of sin. (78)