Why did Abraham lie to Pharaoh about Sarah being his sister in Gen 12?

Regardless of his faith, Abraham was still a man - he had not yet been perfected: he was still on his walk of progressive sanctification.

The same question could be asked as to why David sinned with Bathsheba, or why Samson fell for Delilah.

And any of countless more examples - the men and women portrayed in the Bible were not perfect (excepting Christ). Even Peter, who had been with Jesus since the earliest days of His ministry, denied him three times during the trial before the Sanhedrin.

Every instance is an opportunity to showcase God's faithfulness, mercy, and grace - even when his chosen ones fail Him.

Even those with the stoutest faith in God can forget or become fearful: while backsliding or temporarily turning from God to our own wisdom is never promoted, it is the entire life story that must be examined. When doing that with Abraham, it is obvious that though he was not perfect, he was following God.

First of all, God did protect Abraham:

But the LORD inflicted serious diseases on Pharaoh and his household because of Abram’s wife Sarai.
--Gen 12:17

For the rest of it, it was a matter of Faith on Abraham's part. His faith wavered, and he so he acted in this instance out of fear of man, rather than fear of God.

The Genesis text is widely believed by modern secular scholars to have been put together from various traditions, compiled in at least two major independent narratives, which were merged to produce the final text. The merger is very rough, so that the two narratives may be easily distinguished, because one consistently uses the sacred name "Yahweh" to refer to God, and the other uses the more generic identifier "Elohim"(God) in Genesis and the early parts of Exodus.

To those who doubt this idea, the textual boundaries are sometimes glaringly obvious, as in Exodus 3:14, where the first 14 verses are Elohist, and the rest a Yahweh tack-on. In Exodus 6:2, God says to Moses that he did not reveal his name "Yahweh" to Abraham Isaac and Jacob, only to Moses. But this is contradicted by the Yahwist narrative in Genesis, where all the patriarchs reference Yahweh several times by name. For a nice Genesis example, consider the entire chapter of Genesis 39, which is a lovely Yahwist narrative about Joseph's attempted seduction by his master's wife, which ends with Joseph in jail. This chapter throws off the rest of the narrative, because in the rest, Joseph seems to be Potiphar's servant throughout. The inconsistency is resolved in KJ by a stretched translation--- which claims that Potiphar who is "Rosh Ha-tabachim" is actually the head of the guard. These examples are just those I noticed, but the arguments about this division date back to the 19th century, and it is universally accepted by secular Bible scholars today.

The two narratives have different and recognizably consistent authorial voices, indicative of at least two separate authors, both excellent writers. By the end of Exodus and throughout Leviticus, there is a third much less inspired author writing, who uses a legalistic language, full of unnecessary repetitions and pedantic double-speak, and narration, when it comes, is jarringly bad (like the first verses of Leviticus 10). This author is referred to as "Priestly". Aside from J,E, and P, as the Yahwist, Elohist, and Priestly authors are called, I could not clearly distinguish any other voices, although the academics sometimes do.

The narrative in Genesis 20 is an Elohist narrative, and it is just repeating a different tradition regarding the narrative in Genesis 12, which is Yahwist. The two narratives come from the same tale, but they differ in a few details. The distinguishing details are revealing, since it is here that one learns the most about Abram's relation to Sarai. In the first narrative, Abraham tells pharaoh that he was lying about Sarai being his sister. In the second narrative, Abraham says that Sarah is his half-sister.

This is very strange, since incestuous ancestry is most often used in the pentateuch to indicate that a certain tribe is somehow cursed, or weakened, or inferior. Here, the implied incest is for the ancestors of the Hebrews, so it is really very striking--- it suggests that there was a deep rooted tradition for Abram/Sarai being both brother/sister and husband/wife, a tradition that was able to survive transmutation through many retellings, even with the strong incest taboo that is evident in other parts of Genesis (Gen 35:21 & 49:4 , 19:30-38)

On the internet, one finds a possible explanation. Abraham/Abram and Sarah/Sarai have a very striking parallel in Hindu mythology in the couple Brahma and Saraiswati, who are both brother and sister and husband and wife. The narrative parallel here has led some of the internet folks to suggest that the origin of the Hebrew religion is as a monotheistic offshoot of Hinduism. According to this book, http://books.google.com/books?id=1qBTNVydxCAC&pg=PA872&lpg=PA872&dq=brahma+saraiswati&source=bl&ots=RTd0X0kfWD&sig=fg1l6TnkSxN1U-m95oTdDBGIGP0&hl=en&sa=X&ei=dtQHT7_MKMXs0gHh-LCxAg&sqi=2&ved=0CFkQ6AEwBg#v=onepage&q=brahma%20saraiswati&f=false , Jesuits suggested the connection.

There is another monotheistic tradition which comes from the Persian region which is identified as Abraham's birthplace, which is Zoroastrianism. The existence of different monotheistic traditions claiming to come from the same place suggests a common ancestry, and it could be a monotheistic Brahma cult. If you google "Brahma Saraiswati" you find lots of websites.

The sister/wife story is repeated a third time, with Isaac and Rebekah as the couple. This third repetition is in Genesis 26:7 and thereabouts. The third repetition is not as salient as the other two, but it suggests that different tribes assigned the same stories to different patriarchs. This uses Yahweh as God's identifier.

Um... Abraham didn't lie. The word "sister" can also mean "kinswoman", and Sarah was, in fact, a kinswoman to Abraham (see Gen. 20:12) (this is an example of bearing false witness by telling the truth).

Now, why did he allow them to be deceived? Why didn't he say things outright? In addition to the fact that he was a human being flawed and struggling with sin, it was also the time of Sodom and Gomorrah (chapter 19 is right between the covenant of 17 and the repeat problem in 20). He could have rightly assumed that by wandering into an enemy camp he was signing his own death warrant. While we might enjoy condemning him for these failures in faith, we can't exactly say that we would have done better.