"Let no man deceive himself . . ." (1 Cor. 3:18). "For if anyone thinks he is something when he is nothing, he deceives himself." (Gal. 6:3). "But prove yourselves doers of the word, and not merely hearers who delude themselves." (Jam. 1:22). "If anyone thinks himself to be religious, and yet does not bridle his tongue but deceives his own heart, this man's religion is worthless." (Jam. 1:26).
Of all forms of deception, self-deception is the most deadly, and of all deceived persons the self-deceived are the least likely to discover the fraud. When a man is deceived by another, he is deceived against his will -- temporarily the victim of the other's guile. Since he expects his foe to take advantage of him, he is watchful and quick to suspect trickery. Under such circumstances it is possible to be deceived sometimes and for a short while, but because the victim is resisting he may break out of the trap and escape before too long.
With the self-deceived it is quite different. He is his own enemy and is working a fraud upon himself. He wants to believe the lie and is psychologically conditioned to do so. He does not resist the deceit but collaborates with it against himself. There is no struggle, because the victim surrenders before the fight begins. He enjoys being deceived. The deeply religious man is far more vulnerable than the easygoing fellow who takes his religion lightly. The latter may be deceived, but he is not likely to be self-deceived. (A.W. Tozer)
Self-deception evidences itself in two ways: An overestimation of self-worth and an underestimation of God's revelation. This type of pride and disrespect sets the stage for every other lie to gain a foothold -- regardless of the level of education, culture, or training. But not only that, if persisted in, God simply gives the self-deceived over to what they need least and want most -- more deception (Rom. 1:22.; 2 Thess. 2:11).
Bowing to God's word in humility is not only the first step in salvation, but also the first step toward sanity (Eccl. 9:3). Right thinking always starts with God. Hell is full of people who start with their own opinion and worth: "Jesus said to them, 'If you were blind, you would have no sin; but since you say, "We see," your sin remains'" (Jn. 9:41).