In the Old Testament, “son of God” is a title given to the angels, the Chosen People, the children of Israel, and their kings. It signifies an adoptive sonship that establishes a relationship of particular intimacy between God and his creature. When the promised Messiah-King is called “son of God”, it does not necessarily imply that he was more than human, according to the literal meaning of these texts. Those who called Jesus “son of God”, as the Messiah of Israel, perhaps meant nothing more than this.
Such is not the case for Simon Peter when he confesses Jesus as “the Christ, the Son of the living God”, for Jesus responds solemnly: “Flesh and blood has not revealed this to you, but my Father who is in heaven.” Similarly Paul will write, regarding his conversion on the road to Damascus, “When he who had set me apart before I was born, and had called me through his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles…” “and in the synagogues immediately [Paul] proclaimed Jesus, saying, ‘He is the Son of God.'” From the beginning this acknowledgment of Christ’s divine sonship will be the centre of the apostolic faith, first professed by Peter as the Church’s foundation.
Peter could recognize the transcendent character of the Messiah’s divine sonship because Jesus had clearly allowed it to be so understood. To his accusers’ question before the Sanhedrin, “Are you the Son of God, then?” Jesus answered, “You say that I am.” Well before this, Jesus referred to himself as “the Son” who knows the Father, as distinct from the “servants” God had earlier sent to his people; he is superior even to the angels. He distinguished his sonship from that of his disciples by never saying “our Father”, except to command them: “You, then, pray like this: ‘Our Father'”, and he emphasized this distinction, saying “my Father and your Father”.
The Gospels report that at two solemn moments, the Baptism and the Transfiguration of Christ, the voice of the Father designates Jesus his “beloved Son”. Jesus calls himself the “only Son of God”, and by this title affirms his eternal pre-existence. He asks for faith in “the name of the only Son of God”. In the centurion’s exclamation before the crucified Christ, “Truly this man was the Son of God”, that Christian confession is already heard. Only in the Paschal mystery can the believer give the title “Son of God” its full meaning.
After his Resurrection, Jesus’ divine sonship becomes manifest in the power of his glorified humanity. He was “designated Son of God in power according to the Spirit of holiness by his Resurrection from the dead”. The apostles can confess: “We have beheld his glory, glory as of the only Son from the Father, full of grace and truth.”
From the Catechism of the Catholic Church 441-445