VATICAN CITY (CNS) — Pope Benedict XVI asked Catholics for their prayers as he began his weeklong Lenten retreat March 13.

Before reciting the Angelus prayer at midday with visitors in St. Peter’s Square, he also prayed that Mary would intercede to help everyone have a Lent that is “rich in the fruits of conversion.”

Carmelite Father Francois-Marie Lethel, a theology professor and the secretary of the Pontifical Academy of Theology, was chosen to preach the retreat March 13-19 for Pope Benedict and top Vatican officials. The French priest’s topic was to be “The Light of Christ in the Heart of the Church: John Paul II and the Theology of Saints.”

In his main Angelus address, Pope Benedict discussed the Gospel story of the temptation of Jesus in the desert.

The Pope said the story is a reminder that evil and sin really do exist.

In modern societies where people pretend that God does not exist, it is logical that they do not think there is such a thing as sin or evil either, he said.

Just as there are only shadows when the sun or another light is shining, “so the eclipse of God necessarily leads to the eclipse of sin,” the Pope said.

God sent his only Son to take on human flesh and then to die for the salvation of all, he said, but “the devil, with all his strength, opposes this plan of definitive and universal salvation.”

Setting out on the Lenten journey of conversion, the Pope said, Christians make a commitment “to take Jesus’ side against sin and, as individuals and as a Church, to engage in spiritual battle against the spirit of evil.”