John Gill 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)

And he that saw it, bare record
Meaning himself, John the evangelist, the writer of this Gospel, who, in his great modesty, frequently conceals himself, under one circumlocution or another; he was an eyewitness of this fact, not only of the piercing of his side with a spear, but of the blood and water flowing out of it; which he saw with his eyes, and bore record of to others, and by this writing; and was ready to attest it in any form it should be desired:

and his record is true ;
though it is not mentioned by any of the other evangelists, none of them but himself being present at that time: and he knoweth that he saith true ;
meaning either God or Christ, who knew all things; and so it is a sort of appeal to God or Christ, for the truth of what he affirmed, as some think; or rather himself, who was fully assured that he was under no deception, and was far from telling an untruth; having seen the thing done with his eyes, and being led into the mystery of it by the Divine Spirit; see (1 John 5:6 1 John 5:8) wherefore he could, and did declare it with the strongest asseverations:

that ye might believe ;
the truth of the fact, and in Christ, both for the expiation of the guilt of sin, and cleansing from the filth of it; both for sanctifying and justifying grace, which the water and the blood were an emblem of.