Human nature will try to reassert its dominance in a converted person's life. The word that is translated from Hebrew into the English word "deceitful" means "tortuous, twisted, convoluted." That is the way human nature is—tortuous, twisted, and convoluted in its thinking and reasoning. God is referring to the kind of justifications or rationalizations that it will make to convince a person that it is okay to sin, not to pay attention to the law of God, to say to the self, "It really does not matter. I am only one person."
Note this frank quote from Aldus Huxley, which appears in his book Ends And Means (p. 270):
I had motives for not wanting the world to have a meaning; consequently assumed that it had none, and was able without any difficulty to find gratifying reasons for this assumption.
Convoluted, tortuous, twisted. Looking for rationalizations and justifications. Continuing on page 273:
Most ignorance is vincible ignorance. We don't know because we don't want to know. It is our will that decides how and upon what subjects we shall use our intelligence. . . . Those who detect no meaning in the world generally do so because for one reason or another it suits their books that the world should be meaningless. . . . We objected to the morality because it interfered with our sexual freedom.
This is how human nature works—the heart twists the truth of God and deceives itself into thinking that, "Well, let us make the assumption that the world and life have no meaning, therefore we can do what we want to do." This is from a highly intelligent human being.
We know that for us to acknowledge and yield to truth will be personally costly, and we often do not want to pay the price, so we allow ourselves to be deceived into thinking it really does not matter if we yield to the wrong thing this time. We will fight the battle the next time it comes up.