And he that saw it bare record, and his record is true: and he knoweth that he saith true, that ye might believe.

Wesley's Notes

19:35 His testimony is true - Valid, unexceptionable. And he knoweth - And his conscience beareth him witness, that he testifieth this for no other end, than that ye may believe.

Jamieson-Fausset-Brown Bible Commentary

35. And he that saw it bare record-hath borne witness.

and his witness is true, and he knoweth that he saith true, that ye might believe-This solemn way of referring to his own testimony in this matter has no reference to what he says in his Epistle about Christ's "coming by water and blood" (see on [1913]1Jo 5:6), but is intended to call attention both to the fulfilment of Scripture in these particulars, and to the undeniable evidence he was thus furnishing of the reality of Christ's death, and consequently of His resurrection; perhaps also to meet the growing tendency, in the Asiatic churches, to deny the reality of our Lord's body, or that "Jesus Christ is come in the flesh" (1Jo 4:1-3).

Matthew Henry's Concise Commentary

19:31-37 A trial was made whether Jesus was dead. He died in less time than persons crucified commonly did. It showed that he had laid down his life of himself. The spear broke up the very fountains of life; no human body could survive such a wound. But its being so solemnly attested, shows there was something peculiar in it. The blood and water that flowed out, signified those two great benefits which all believers partake of through Christ, justification and sanctification; blood for atonement, water for purification. They both flow from the pierced side of our Redeemer. To Christ crucified we owe merit for our justification, and Spirit and grace for our sanctification. Let this silence the fears of weak Christians, and encourage their hopes; there came both water and blood out of Jesus' pierced side, both to justify and sanctify them. The Scripture was fulfilled, in Pilate's not allowing his legs to be broken, Ps 34:20. There was a type of this in the paschal lamb, Ex 12:46. May we ever look to Him, whom, by our sins, we have ignorantly and heedlessly pierced, nay, sometimes against convictions and mercies; and who shed from his wounded side both water and blood, that we might be justified and sanctified in his name.