There is a theory developed in the late 1800's called the “Documentary Hypothesis”. According to this theory, the first five books of the Bible (or “Pentateuch”), which Christians have traditionally believed were written by Moses, were probably in fact written by various authors over 1000 years or more. Let's go over this idea to get a feel for why one might believe this. Firstly, the end of the book of Deuteronomy records the death and burial of Moses; so that clearly wasn't written by Moses. Advocates of the theory also wonder why the author would use the term “Yahweh” for God in some places, but “Elohim” in others. Surely this suggests that at least two different authors were involved, each having a different writing style? People also wonder why there are two Creation accounts, and apparent discrepancies in the flood account, for example, as though two different flood accounts had been blended together. Lastly, the books of the Bible which supposedly come later don’t seem to make any citations of the Pentateuch, though it is considered such a foundational document.
In response to the first issue I have mentioned; (that Deuteronomy records the death of Moses), we have to confess quite frankly that not every word of the Pentateuch was literally written by Moses. At least this chapter would have been added by someone else, and it is commonly believed that it was added by Moses' successor, Joshua. We could also find other tell-tale signs that the Pentateuch has been edited since it's first draft. For example, in Genesis 14:14 it says that Abraham pursued his enemies “as far as Dan.” Dan was a city which wasn't built until the time of Judges chapter 18. Dan represented the northern most part of Israel, so to say that Abraham pursued his enemies “as far as Dan” is a way of saying that he pursued them to the very border of Israeli territory, which in Abraham's time was the land of Canaan. Before such an edit, the Text may have simply used some other way of referring to the border. Such edits do not necessarily lead us to believe that Moses did not write the original Text, or that the Text we have now is radically different from the original.
On the use of the term Yahweh versus Elohim, this seems to me to be the weakest argument. The Hebrew word “Elohim” simply means “God”, whereas the term “Yahweh” is the name of God. We might think little of it that Moses sometimes chooses to use the name of God and sometimes chooses to simply say “God”, except that we know from history that in Jewish culture, around the time of the Babylonian exile, it became a sort of taboo to use the name of God, “Yahweh”. Instead, Jews to this day will typically use the term “Adonai”, which means “Lord.” So, according to the theory, the authorship of the Pentateuch is divided up according to the term used; those portions in which the word “Elohim” is used are supposedly written by a later author, not wanting to use the word “Yahweh”. But this seems like a very tenuous argument. In fact, no two advocates of the Documentary Hypothesis agree on which parts were written by different authors, which makes the distinction based on term usage highly questionable. But if the Pentateuch were so edited by someone during the Babylonian captivity, why didn't they simply change all the references from “Yahweh” to “Elohim”? It can't have been out of reverence for the Text that they left them in, since they are essentially butchering it anyway. Deuteronomy itself tells us: “Everything that I command you, you shall be careful to do. You shall not add to it or take from it.” (Deuteronomy 12:32). Any prophet who thought they were writing Scripture would not have approved of a redaction of the Pentateuch.
People believe the Pentateuch was all written in hind-sight in order to explain the situation of Israel after the Babylonian captivity. So, on the apparent lack of references to the Pentateuch in the rest of the Old Testament books, it is actually quite important to see how the Pentateuch does in fact underpin all of the Old Testament books. The Pentateuch establishes the Covenant, or describes the formal relationship that Israel was to have with God. God established the Law in the Pentateuch, and told Israel that they would suffer various consequences for disobedience, such as famine and war, and eventually exile from the land. What's more, we read in the Pentateuch that God would establish prophets whose job was to remind Israel of this Covenant whenever they would stray from it. The very fact that we have prophets in the other Old Testament books, and the things that those prophets write about; namely exile for disobedience... all of this is precisely a direct allusion to the Pentateuch! It's difficult, for example, to put Nehemiah chapter 9 in the context of a Pentateuch written later than, say, King David. Chronologically speaking, Nehemiah is one of the last books of the Old Testament, itself written post-exile. In chapter 9, the history of Israel is outlined as starting with Moses and resulting in exile because of disobedience to the Law that God gave through Moses. In chapter 8 it is even said that a particular Law had not been observed since the time of Joshua. This really doesn't make sense if the Pentateuch was still "hot off the press", so to speak.
What the Documentary Hypothesis ultimately concludes is that the entire religion of the Jews was “made up” during the exile in order to explain why they had been conquered. For Nehemiah to cite Genesis and Deuteronomy, and to refer to Joshua, one must conclude, if they are determined to hold to the Documentary Hypothesis, that none of these people really existed and none of those events really took place. Yet most of the prophets in the Old Testament speak about the exile in a future-tense, predicting that it will come because of Israel's disobedience to the Law – the Law that was established “past-tense” in the Pentateuch. Dividing the Pentateuch up this way simply doesn't make sense to me. But if we believe in the authority of Christ Himself, we know that Moses wrote the Pentateuch in his own lifetime, since Jesus attributes it to Moses continually. And if you take the divisions of the Pentateuch, according to theses scholars, you can even find Jesus attributing sections from each of these divisions to Moses! I leave you with this...
Then he said to them, “These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.” Then he opened their minds to understand the Scriptures (Luke 24:44-45)