A.T. Robertson Commentary John 19:35

"And he that hath seen hath borne witness, and his witness is true: and he knoweth that he saith true, that ye also may believe." — John 19:35 (ASV)

He that hath seen (ο εωρακως). Perfect active articular participle of οραω. John the Apostle was there and saw this fact (still sees it, in fact). This personal witness disproves the theory of the Docetic Gnostics that Jesus did not have a real human body.

He knoweth (εκεινος οιδεν). That is John does like 9:37. It is possible that εκεινος may be a solemn appeal to God as in 1:33 or Christ as in 1 John 3:5. Bernard argues that the final editor is distinguishing the Beloved Disciple from himself and is endorsing him. But the example of Josephus (War. III. 7, 16) is against this use of εκεινος. John is rather referring to himself as still alive.