The one who saw it has testified, and his testimony is true. He knows he is telling the truth, so that you also may believe.
In context
33But when they came to Jesus and saw that he was already dead, they did not break his legs. 34Instead, one of the soldiers pierced his side with a spear, and at once blood and water flowed out. 35The one who saw it has testified, and his testimony is true. He knows he is telling the truth, so that you also may believe. 36These things happened so that the Scripture would be fulfilled: “Not one of his bones will be broken.” 37And again another Scripture says, “They will look on the one they pierced.”