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. )