John Gill Commentary John 5:47

"But if ye believe not his writings, how shall ye believe my words?" — John 5:47 (ASV)

But if you believe not his writings
They believed them to be his writings, and that they were the word of God, and yet did not believe the things contained in them, respecting Christ; or did not see, and could not believe that they belonged to, and were applicable to Jesus of Nazareth; and therefore it could not be supposed they would give credit to him, or his words:

how shall you believe my words ?
not that Moses was greater than Christ, or rather to be credited than he; Moses indeed was faithful, but Christ was worthy of more honour and credit than he was; Moses was but a servant, but Christ was a son in his own house: but this is said with respect to the Jews, with whom Moses was in great veneration and esteem; and it was more likely they should regard what he should say, than what Jesus of Nazareth should, whom they despised.