Although many people reject the notion of sin, it is a reality of life, said Pope Benedict XVI at Sunday's Angelus.
He encouraged believers to join with Christ in "spiritual combat" during Lent.
Despite a steady rain, thousands of pilgrims were present in St. Peter's Square to pray the Angelus with the Pope on March 13, the first Sunday of Lent.
A group of motorcyclists greeted him from the street with a chorus of honks.
The 40 days of Lent constitute a time of spiritual preparation for Easter, of following Jesus to the "culmination of his mission of salvation" in his death on the cross, said the Pope.
Lent and the cross exist "because evil exists," he said. And although many do not accept the term "sin" for offering a religious vision of the world and humanity, sin is "the profound cause of every evil," he explained.
"In fact," said the Pope, "it is true: if God is eliminated from the horizons of the world, one can no longer speak of sin."
He compared humanity's sense of sin to a shadow that only exists in the presence of the sun, and disappears when the sun is hidden. In such a way, he said, "the eclipse of God brings with it necessarily the eclipse of sin."
"Therefore the sense of sin - which is different from the 'sense of guilt' as psychology understands it - is acquired, rediscovering the sense of God."
The Pope said that King David's Psalm 51, a prayer of repentance written after he committed both adultery and homicide, expresses this sense.
"Against you only have I sinned," David tells God.
God's attitude is one of opposing the sin while saving the sinner, said the Pope. "God does not tolerate evil, because he is love, justice and fidelity – and precisely for this he does not want the death of the sinner, but that he may repent and live."
He observed that God's saving intervention in human history has been evident from the time of the ancient Jews' liberation from slavery in Egypt. "God is determined to liberate his children from slavery," he reflected, "to guide them to freedom."
"And the most serious and most profound slavery is precisely that of sin. For this, God sent his son to the world: to liberate men from the dominion of Satan, 'origin and cause of every sin'."
"He sent him in our mortal flesh so that he might become a victim of expiation, dying for us on the cross."
"Against this plan of definitive and universal salvation, the devil is opposed with all his strength, as demonstrated particularly in the Gospel of the temptation of Jesus in the desert, which is proclaimed every year on the first Sunday of Lent," said the Pope.
"In fact, entering into this liturgical time means aligning oneself with Christ every time against sin – facing, both as individuals and the Church, the spiritual combat against the spirit of evil."
Pope Benedict invoked the assistance of Mary so that Lent might be "rich with the fruits of conversion."
He asked for special prayer for himself and members of the Roman Curia as they begin their Lenten exercises.
Carmelite theologian and professor, Fr. Francois-Marie Lethel, is leading the March 13-19 exercises this year on the theme "The Light of Christ in the Heart of the Church - John Paul II and the Theology of the Saints."
After the Angelus prayer, the Pope remembered the victims of the earthquake and consequent tsunami this week in Japan.
He encouraged those working in emergency response and asked the faithful to join him in praying for the victims.