Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight." Sin sets itself against God's love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods," knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God." In this proud self-exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.
Source: Catechism of the Catholic Church (Vatican.va)