The reality of sin
Sin is present in human history; any attempt to ignore it or to give this dark reality other names would be futile. To try to understand what sin is, one must first recognize the profound relation of man to God, for only in this relationship is the evil of sin unmasked in its true identity as humanity's rejection of God and opposition to him, even as it continues to weigh heavy on human life and history.
Source: Catechism of the Catholic Church (Vatican.va)