Catechism of the Catholic Church

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).