A.T. Robertson Commentary John 5:31

"If I bear witness of myself, my witness is not true." — John 5:31 (ASV)

If I bear witness of myself (Εαν εγω μαρτυρω περ εμαυτου). Condition of third class, undetermined with prospect of determination (εαν and present active subjunctive of μαρτυρεω). The emphasis is on εγω (I alone with no other witness).

Is not true (ουκ εστιν αληθης). In law the testimony of a witness is not received in his own case (Jewish, Greek, Roman law). See De 19:15 and the allusion to it by Jesus in Mt 18:16. See also 2 Corinthians 13:1; 1 Timothy 5:19. And yet in 8:12-19 Jesus claims that his witness concerning himself is true because the Father gives confirmation of his message. The Father and the Son are the two witnesses (8:17). It is a paradox and yet true. But here Jesus yields to the rabbinical demand for proof outside of himself. He has the witness of another (the Father, 5:32,37), the witness of the Baptist (5:33), the witness of the works of Jesus (5:36), the witness of the Scriptures (5:39), the witness of Moses in particular (5:45).