Sin came into the world through the devil, who first fell away from God. Sin then came also through human beings, who, tempted by the devil, voluntarily succumbed to sin.
Joh. 8:44: He (the devil) was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own, for he is a liar, and the father of it.
1 Joh.3:8: He that committeth sin is of the devil, for the devil sinneth from the beginning.
Rom. 5:12: By one man sin entered into the world, and death by sin.
Assignment: Read Gen. 3:1-7: the fall into sin.