Man's first sin
In that sin man preferred himself to God and by that very act scorned him. He chose himself over and against God, against the requirements of his creaturely status and therefore against his own good. Created in a state of holiness, man was destined to be fully "divinized" by God in glory. Seduced by the devil, he wanted to "be like God", but "without God, before God, and not in accordance with God".
Source: Catechism of the Catholic Church (Vatican.va)