But if ye believe not his writings, how shall ye believe my words?

People's New Testament

5:45-47 There is one that accuseth you. Moses, whose testimony they failed to accept. If they rejected the testimony of Moses, whom they professed to reverence, how could they believe him of whom Moses spoke?

Jamieson-Fausset-Brown Bible Commentary

47. If ye believe not-(See Lu 16:31).

his writings . my words-a remarkable contrast, not absolutely exalting Old Testament Scripture above His own words, but pointing to the office of those venerable documents to prepare Christ's way, to the necessity universally felt for documentary testimony in revealed religion, and perhaps (as Stier adds) to the relation which the comparative "letter" of the Old Testament holds to the more flowing "words" of "spirit and life" which characterize the New Testament.

Matthew Henry's Concise Commentary

5:45-47 Many trust in some form of doctrines or some parties, who no more enter into the real meaning of those doctrines, or the views of the persons whose names they bear, than the Jews did into those of Moses. Let us search and pray over the Scriptures, as intent on finding eternal life; let us observe how Christ is the great subject of them, and daily apply to him for the life he bestows.