Adam, although perfect, let his heart be enticed; he rejected the truth and turned away from God. (See Jas 1:14, 15.) Consequently, all humans, the offspring of fallen Adam, have been conceived in sin and brought forth in error. (Ps 51:5) After the Flood, God said regarding sinful mankind in general: “The inclination of the heart of man is bad from his youth up.”—Ge 8:21.
God told the rebellious nation of Judah: “The heart is more treacherous than anything else and is desperate.” (Jer 17:9) This constitutes a serious warning that those seeking to please God must give attention not merely to what other humans see but to the kind of person they really are, the inner man. A person may have been a Christian for many years, have a fine knowledge of the Bible, and feel confident that he can safely handle any situation that may arise. Yet, although he knows full well that an act is wrong and specifically condemned by God’s law, the thoughts and desires that he has secretly cherished may entice him into sinful action.
For these reasons a Christian, though he knows the truth and may consider himself mature, must remember the treachery that his heart can display and must therefore exercise great care not to place himself in the way of temptation.—Mt 6:13; 1Co 10:8-12.