John 19:35

Lest the reader miss the point of John 19:34, John explained that he had personally witnessed what he narrated and that he was not lying. Furthermore the purpose of his reliable eyewitness testimony was that his readers might believe what he wrote and what it meant, namely, that Jesus was God’s Son (cf. John 20:30-31; John 21:24).

Some commentators suggested that the eyewitness was someone different from John. Suggestions range from the soldier who pierced Jesus’ side, to an unknown eyewitness whom John did not identify, to an unknown editor, to Jesus, and to God the Father. However the most probable solution is to identify John himself as the eyewitness in view of the context and the parallel statements that follow (John 20:30-31; John 21:24; cf. John 1:14; John 12:23).