From the Catechism of the Catholic Church, Simplified
Man's Need to Be Tested (396)
Man had to freely accept God's friendship. God's command to Adam (not to eat of the tree of knowledge of good and evil) shows symbolically that man had to acknowledge his limits as a creature and submit his behavior to God's laws.
The First Sin (397)
By sin, man set aside his trust in God and disobeyed. All subsequent sin would be disobedience and lack of trust. By sin, man rebelled against his status as a creature. Man chose to be like God "without God and not in accordance with God" (St. Maximus).
What They Lost (398-399)
Our first parents lost original holiness and became afraid of God, whom they falsely thought was jealous of his prerogatives. The soul's control over the body's powers was shattered. The union of man with woman became subject to tensions and their sexual relationship was marked by lust and domination (Gen 3:5-16).
Creation became hostile to man and death made its entrance into human history. "Sin entered the world and through sin, death..." (Rom 5:12).
A History of Sin (400-401)
Sin inundated human history and even entered God's Covenant with Israel. Even after Christ's death, sin was present in the Christian community. There is a universality of sin.
This is confirmed by our own experience. "Often refusing to acknowledge God, man has broken the right order that should reign within himself and with other creatures" (Second Vatican Council).
Everyone Affected (402-403)
All are implicated in sin. "Death spread to all men because all men sinned" (Rom 5:12). However, all are offered salvation in Jesus Christ. "One man's act of righteousness leads to acquittal and life for all men" (Rom 5:18).
This overwhelming misery which oppresses is connected with the fact that Adam's sin has been transmitted to all at birth. This sin is the "death of the soul." Therefore, the Church baptizes even tiny infants "for the remission of sins" (Council of Trent).
Transmission of Effects (404)
We do not exactly know how this "original sin" is transmitted. Adam and Eve were supposed to transmit original holiness and justice to everyone. In their "fallen state" they could only pass on a human nature deprived of original holiness and justice. Therefore, original sin differs from other sins. It is contracted (a state) not committed (an act).
Deprived of Our Heritage (405)
By original sin, we are deprived of original holiness and justice. Human nature is not totally corrupted but is wounded in its natural powers. It is subject to ignorance, sin, and death, and has an inclination to evil (called "concupiscence"). Baptism erases original sin and turns the person back to Christ. However, the inclination to sin that persists requires that man be in a constant, spiritual battle.
Two Errors (406)
Pelagius (5th century) reduced the influence of Adam's fault to bad example and taught that man could lead a good life without God's help. In contrast, the first Protestant teachers (16th century) taught that original sin had radically permeated man, and man's tendency to evil was insurmountable. The Church taught against Pelagius at the Council of Orange and against Protestant teachers at the Council of Trent.
Today's Battle (407)
Original sin provides great insight into man's activity in the world. Although remaining free, man has come under "the devil, who has the power of death" (Council of Trent). Man is ignorant of his own inclination to evil and makes serious errors in education, politics, and social action.
A Battlefield Due to Sin (408-409)
Original sin and all personal sins place the world in a sinful condition. This "sin of the world" (Jn 1:29) refers also to the negative influences exerted by sinful social structures. A dramatic situation exists in which "the whole world is in the power of the evil one" (1 Jn 5:19). Man is on a battlefield and only by God's aid does he succeed in achieving his own inner integrity (Second Vatican Council).
A Promised Victory (410-411)
After original sin, God still called man and even promised a victory. He announced that there would be a Messiah and Redeemer, a battle between the serpent and the Woman, and the final victory for her descendents: "I will put an enmity between you and the woman and between your offspring and hers, he will strike at your head while you strike at his heel" (Gen 3:15).
This passage announces Christ as the "New Adam" who superabundantly makes amends for Adam's sin. Many church Fathers see Mary as the "new Eve," the first to benefit from Christ's redemption by her Immaculate Conception and freedom from all personal sins.
Why Was Sin Permitted? (412)
Many Christian writers answered that question. "Christ gave us better blessings than the devil took away" (Leo the Great). "There is nothing to prevent human nature from being raised up to something greater, even after sin" (Thomas Aquinas). "O happy fault... which gained for us so great a Redeemer" (Easter Vigil Exsultet).