1. the bible never says that moses wrote it (so its not even a challenge if you somehow still believe in inerrancy)
2. Most scholars studying WITHOUT the preconceived notion that Moses wrote them all find several reasons to conlcude multiple authorship over a period of time:
a. Contrasting Couplet Stories
There are several instances where the same story is told twice in different ways. They believe this is because someone gathered old writings from several sources and organized them. Some examples:
Two creation stories in Genesis.
Two descriptions of the Abrahamic covenant.
Two stories of the naming of Isaac.
Two instances where Abraham deceived a king by introducing his wife Sarah as his sister.
Two stories of Jacob traveling to Mesopotamia
Two stories of a revelation at Beth-el to Jacob.
Two accounts of God changing Jacob's name to Israel
Two instances where Moses extracted water from two different rocks at two different locations called Meribah.
Genesis 11:31 This describes Abraham as living in the city Ur, and associates that location with the Chaldeans. Archeological evidence indicates that the Chaldeans did not exist as a tribe at the time of Abraham; they rose to power much later, during the 1st millennium BCE.
this appears to indicate that the story was written by someone AFTER the events, transmuting familiar peoples and places into the oral tradition. This happens other times:
Genesis 14:14: This verse refers to Abram pursuing some surviving kings of Sodom and Gomorrah to the city of Dan. However, that place name did not exist until a long time after Moses' death. Other locations are also identified in the Pentateuch by names that were invented long after the death of Moses.
Genesis 22:14: The verse states: "And Abraham called the name of that place Jehovahjireh: as it is said to this day..." There are many verses in the Torah that state that something has lasted "to this day". That appears to have been written by a writer who composed the passages long after the events described, and long after Moses' death.
Genesis 36 contained a list of Edomite kings which included some monarchs who were in power after Moses' death. R.E. Friedman wrote: "In the eleventh century, Isaac ibn Yashush, a Jewish court physician of a ruler in Muslim Spain, pointed out that a list of Edomite kings that appears in Genesis 36 named kings who lived long after Moses was dead. Ibn Yashush suggested that the list was written by someone who lived after Moses. The response to his conclusion was that he was called "Isaac the blunderer." History has proven him to be correct, at least as viewed by most mainline and liberal theologians.
Deuteronomy 34:10 This states "There has never been another prophet like Moses..." (NLT) This sounds like a passage written long after Moses' death. Enough time would have had to pass for many other prophets to have arisen, to passed from the scene, and to have been evaluated.
check out
http://www.religioustolerance.org/chr_tora.htm to read up on this... there's no reason to think that Moses DID write the entire thing besides tradition anyways