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.
Source: Catechism of the Catholic Church (Vatican.va)