3 “And he that saw it,” he says, “bare record, and his record is true; and he knows that he says true, that you also might believe.” He said not, That ye also might know, but “that you might believe;” for he knows who has seen, that he who has not seen might believe his testimony. And believing belongs more to the nature of faith than seeing. For what else is meant by believing than giving to faith a suitable reception? “For these things were done,” he adds, “that the scripture should be fulfilled, A bone of Him you shall not break.
And again, another scripture says, They shall look on Him whom they pierced.” He has furnished two testimonies from the Scriptures for each of the things which he has recorded as having been done. For to the words, “But when they came to Jesus, and saw that He was dead already, they broke not His legs,” belongs the testimony, “A bone of Him you shall not break:” an injunction which was laid upon those who were commanded to celebrate the passover by the sacrifice of a sheep in the old law, which went before as a shadow of the passion of Christ.
Whence “our passover has been offered, even Christ,” of whom the prophet Isaiah also had predicted, “He shall be led as a lamb to the slaughter.” In like manner to the words which he subjoined, “But one of the soldiers laid open His side with a spear,” belongs the other testimony, “They shall look on Him whom they pierced;” where Christ is promised in the very flesh wherein He was afterwards to come to be crucified.
Source: Tractates on the Gospel of John (New Advent)