35 The man who saw it has given testimony, and his testimony is true. He knows that he tells the truth, and he testifies so that you also may believe.

Read John 19:35 Using Other Translations

And he that saw it bare record, and his record is true: and he knoweth that he saith true, that ye might believe.
He who saw it has borne witness--his testimony is true, and he knows that he is telling the truth--that you also may believe.
(This report is from an eyewitness giving an accurate account. He speaks the truth so that you also may continue to believe. )