Albert Barnes on John 19:35

He that saw it. John himself. He is accustomed to speak of himself in the third person.

His record is true. His testimony is true. Such was the known character of this writer, such his sacred regard for truth, that he could appeal to that with full assurance that all would put confidence in him. He often appeals thus to the fact that his testimony was known to be true. It would be well if all Christians had such a character that their word would be assuredly believed.

(i) "And he that saw" Jn 1:1-3