But if you do not believe his writings. Christ appears here to claim less authority for himself than for Moses; and yet we know that heaven and earth have been shaken by the voice of the Gospel, (Hebrews 12:26.) But Christ accommodates his discourse to those to whom he speaks; for the authority of the Law was, beyond all controversy, held sacred among the Jews; and thus it was impossible that Christ should be inferior to Moses. To the same purpose is the contrast between writings and words; for he shows their unbelief to be more aggravated, because the truth of God, recorded in an authentic form, has no authority with them.

If ye believe not his writings - If you do not credit what he has written which you profess to believe, it is not to be expected that you will believe my declarations. And from this we may learn:
1. That many men who profess to believe the Bible have really no regard for it when it crosses their own views and inclinations.
2. It is our duty to study the Bible, that we may be established in the belief that Jesus is the Messiah.
3. The prophecies of the Old Testament are conclusive proofs of the truth of the Christian religion.
4. He that rejects one part of the Bible, will, for the same reason, reject all.
5. The Saviour acknowledged the truth of the writings of Moses, built his religion upon them, appealed to them to prove that he was the Messiah, and commanded men to search them. We have the testimony of Jesus, therefore, that the Old Testament is a revelation from God. He that rejects his testimony on This subject must reject his authority altogether; and it is vain for any man to profess to believe in the New Testament, or in the Lord Jesus, without also acknowledging the authority of the Old Testament and of Moses.
We have in this chapter an instance of the profound and masterly manner in which Jesus could meet and silence his enemies. There is not anywhere a more conclusive argument, or a more triumphant meeting of the charges which they had brought against him. No one can read this without being struck with his profound wisdom; and it is scarcely possible to conceive that there could be a more distinct declaration and proof that he was equal with God.

But if ye believe not his writings, etc. - If you lay them not to heart - if you draw not those conclusions from them which their very letter, as well as their spirit, authorizes you to draw, how shall ye believe my words, against which ye have taken up the most ungrounded prejudice? It is no wonder that we find the Jews still in the gall of bitterness, and bond of iniquity: as they believe not Moses and the prophets, in reference to the Messiah, it is no marvel that they reject Christ and the apostles. Till they see and acknowledge, from the law and the prophets, that Christ must have come, they will never believe the Gospel. St. Paul says, 2-Corinthians 3:15, that even until this day, when Moses (i.e. the law) is read, the Veil is upon their hearts: - so that they see not to the end of that which is abolished: 2-Corinthians 3:13. Nor will this veil be taken away, till they turn from worldly gain and atheism (which appears to be their general system) to the Lord, 2-Corinthians 3:16; and then the light of the glory of God shall shine on them in the face (through the mediation and merits) of Jesus Christ.
It appears that this discourse of our Lord had effectually confounded these Jews, for they went away without replying - a manifest proof they had nothing to say.
1. In all periods of their history, the Jews were both an incredulous and disobedient people: perhaps it was on this ground that God first chose them to be keepers of his testimonies; for had they not had the most incontrovertible proofs that God did speak, they would neither have credited nor preserved his oracles.
Their incredulity is, therefore, no mean proof of the Divine authority of the law and the prophets. The apostles, who were all Jews, partook deeply of the same spirit, as various places in the Gospel prove; and, had not they had the fullest evidence of the divinity of their Master, they would not have believed, much less have sealed the truth with their blood. Thus their incredulity is a strong proof of the authenticity of the Gospel.
2. When a man, through prejudice, bigotry, or malevolence, is determined to disbelieve, both evidence and demonstration are lost upon him: he is incapable of conviction, because he is determined not to yield. This was, this is, the case with the Jews - there are facts before their eyes sufficient to convince and confound them; but they have made a covenant with unbelief, and therefore they continue blind, ignorant, and wicked; obstinately closing their eyes against the light; and thus the wrath of God is coming upon them to the very uttermost. But shall not a rebellious and wicked Christian be judged worthy of more punishment? Certainly: for he professes to believe that truth which is able to make him wise unto salvation, by faith in Jesus Christ. Reader, it is an awful thing to trifle with the Gospel! - the God of it is pure, jealous, and holy. Come unto him and implore forgiveness of thy past sins, that thou mayest have eternal life.

But if ye 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 unto, 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 ye 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.

If ye believe not--(See Luke 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.