John 5:31

If I bear witness of Myself, that I am the Son of God, and therefore as man altogether conformed to the judgment and will of God, My witness is not true, that is, legitimate, judicial, worthy of credit. The word true here is not opposed to false, but to untrustworthy, uncertain. It answers to the Hebrew word neeman, faithful, worthy of credit. For it may be that a man may utter most true testimony concerning himself, and yet may fail to gain credit because of a suspicion that he has too great love of himself, as Euthymius says.

There is a prolepsis by which Christ meets a tacit objection of the scribes, to the following effect. "Thou, 0 Jesus, proclaimest Thyself to be the Son of God, and so in all things to follow the judgment of God. But we will not believe Thee unless Thou shalt prove what Thou sayest by the testimony of God, or of men worthy of credit. This testimony of Thine in a matter which peculiarly concerns Thyself appears to us open to suspicion." Jesus replies, "I grant you that My testimony concerning Myself is not legitimate, nor worthy of credit, if I alone bear witness of Myself. I grant therefore that you need not believe Me alone; but I am not alone, but others worthy of credit bear witness of Me, as will appear by what follows." Christ is here speaking of the common opinion of the Jews, not uttering His own sentiments, as appears from chap. viii. 13, where the Jews openly object to Christ, Thou bearest testimony of Thyself, Thy testimony is not true. Then Christ answers, My testimony is true, &c., because I am not alone, but I, and the Father who sent Me. So S. Cyril