John Lightfoot on John 21:24

[And we know that his testimony is true.] The evangelist had said before, chapter 19:35, "He knoweth that he saith true"; and here in this place he changeth the person, saying, "We know that his testimony is true."

I. One would believe that this was an idiotism in the Chaldee and Syriac tongue, to make 'We' know, and 'I' know, the same thing: which is not unusual in other languages also; Joshua 2:9, I know. The Targumist hath which you would believe to be We know. 1 Samuel 17:28, I know. Targumist, We know.

II. We suppose the evangelist, both here and chapter 19:35, referreth to an eyewitness. For in all judicial causes the ocular testimony prevailed. If any person should testify that he himself saw the thing done, his witness must be received: for true when it is said of any testimony, does not signify barely that which is true, but that which was to be believed and entertained for a sure and irrefragable evidence. So that the meaning of these words is this: "This is the disciple who testifies of these things and wrote them: and we all know that such a testimony obtains in all judgments whatever; for he was an eyewitness, and saw that which he testifies."