What is the Pentateuch?
This is the name used to refer to the five books written by Moses: Genesis, Exodus, Leviticus, Numbers, and Deuteronomy.
How do you know that these books were written by Moses?
This is the teaching of Jesus. In all His teaching, Jesus assumed Moses to be the author of the Pentateuch. For example, after His resurrection, He revealed Himself to His disciples. On this occasion, Jesus told them that all the prophecies made about Him in the Old Testament must be fulfilled. The words he uses for the Old Testament are:
Now He said to them, “These are My words which I spoke to you while I was still with you, that all things which are written about Me in the Law of Moses and the Prophets and the Psalms must be fulfilled.” (Luke 24:44)
By “law of Moses” here, Jesus is referring to the Pentateuch.
What does it mean to say that Moses was the author?
It does not mean that Moses wrote every word of the Pentateuch himself. No doubt, Moses would have written some portions of the Pentateuch from his own knowledge of events; others he would have written using a variety of different sources much as the gospel writers did. It is also possible that God revealed parts of the Pentateuch directly to Moses much as God did with the prophets. Wilson writes:
Certainly it [Mosaic authorship] cannot mean that to be the author Moses must have written his literary works with his own hand. Else would Prescott not be the author of the Conquest of Mexico nor Milton of Paradise Lost nor the kings of Egypt, Babylon, Assyria, and Persia of their inscriptions, nor Jesus of the Sermon on the Mount. source
Finally, it is also true that some parts of the Pentateuch were not written by Moses but added in by later editors such as the account of Moses’ death in Deuteronomy 34.
Why do some deny that Moses wrote the Pentateuch?
Because they discern the presence of various source documents in these books and thus assume that the Pentateuch was edited and reedited many times with the final product not being completed until after the return from exile.
Who was the first such scholar?
The first was a Roman Catholic scholar named Father Simon. He made his observations in 1678 in his book Critical History of the Old Testament. He writes:
No one can doubt but that the truths contained in the Holy Scripture are infallible and of Divine Authority since they proceed immediately from God who in this has only made use of the ministry of men to be his Interpreters. So there is no person either Jew or Christian who does not acknowledge that the Scripture being the pure word of God is at the same time the first principle and foundation of Religion. But as Men have been the depositories of these sacred books as well as of all others and their first originals have been lost, it was in some sort impossible but that there must needs happen some changes as well by reason of the length of time as the carelessness of transcribers. source
He drew attention to the fact that there were duplicate accounts of Creation.
For example, can any one believe that an historian should write the history of the creation of man with so little order as there is in the first chapter of Genesis, where the same things are several times repeated without method and as it were besides the purpose? and moreover after the man and the woman were created in the first chapter and 27th verse the woman is supposed not to be made and in the following chapter, the manner how she was taken from Adam’s side is described, nevertheless in the same chapter, it was before forbidden him, as he was her husband, whom she accompanied in the garden to eat the fruit of a certain tree. source
His conclusion was that Moses had written the legal portions of the Pentateuch and other authors had rounded out the rest of the Pentateuch. Simon believe that these men who finished what Moses started were also under the inspiration of God. source He used these observations to refute the protestant principle of the perspicuity of Scripture.
Those Protestants without doubt are either ignorant or prejudiced who affirm that the Scripture is plain of itself. As they have laid aside the Tradition of the Church and will acknowledge no other principle of Religion but the Scripture itself they were obliged to suppose it plain and sufficient for the establishing the truth of Faith without any Tradition; but if we but consider the conclusions which the Protestants and Socinians draw from the same principle, we shall be convinced that their principle is not so plain as they imagine since these conclusions are so different and the one absolutely denies what the other affirms. Instead of believing with the Protestants that the shortest and most certain way of deciding the questions of Faith is to consult the Holy Scriptures we shall on the contrary find in this work that if we join not Tradition with the Scripture we can hardly affirm any thing for certain in Religion. source