CATECHISM OF THE CATHOLIC CHURCH
SECOND EDITION

Paragraphs 454–457

454 The title "Son of God" signifies the unique and eternal relationship of Jesus Christ to God his Father: he is the only Son of the Father (cf. Jn 1:14, 18; 3:16, 18); he is God himself (cf. Jn 1:1). To be a Christian, one must believe that Jesus Christ is the Son of God (cf. Acts 8:37; 1 Jn 2:23).
455 The title "Lord" indicates divine sovereignty. To confess or invoke Jesus as Lord is to believe in his divinity. "No one can say 'Jesus is Lord' except by the Holy Spirit'" (1 Cor 12:3).
456 With the Nicene Creed, we answer by confessing: "For us men and for our salvation he came down from heaven; by the power of the Holy Spirit, he became incarnate of the Virgin Mary, and was made man."
457 The Word became flesh for us in order to save us by reconciling us with God, who "loved us and sent his Son to be the expiation for our sins": "the Father has sent his Son as the Savior of the world", and "he was revealed to take away sins":

Sick, our nature demanded to be healed; fallen, to be raised up; dead, to rise again. We had lost the possession of the good; it was necessary for it to be given back to us. Closed in the darkness, it was necessary to bring us the light; captives, we awaited a Savior; prisoners, help; slaves, a liberator. Are these things minor or insignificant? Did they not move God to descend to human nature and visit it, since humanity was in so miserable and unhappy a state?