João 1:34

John fulfilled this purpose by witnessing that Jesus was the Son of God (cf. 2 Samuel 7:14; Psalms 2:7). This is a title that unambiguously claims deity. The title "Messiah" did not imply deity to many who heard it in Jesus’ day. They thought only of a political deliverer. Even the Twelve struggled with this. However, John the Baptist testified that Jesus was God, though doubts arose in his mind later. Son of God does not mean any less than deity. It means full deity (John 1:18). This verse is the climax of John the Baptist’s testimony concerning Jesus.

The event that identified Jesus as the Son of God for John the Baptist was the fulfillment of God’s promise to him that he would see the Spirit’s descent and continuation on Him. This was the basis of John the Baptist’s witness concerning Jesus.