Some people believe Abraham was at fault concerning his confrontation with Abimelech in Genesis 20. I’ve even read in commentaries that faithful Abraham wasn’t faithful here, and it is a real tragedy when an unbeliever must chastise a believer for doing wrong (cp. Genesis 20:2, 9-10). Yet, if Abraham is being chastised by the Lord through Abimelech, why did the Lord bless him over what was done (Genesis 20:14-16)? Moreover, why did Isaac do the same thing concerning Rebecca (Genesis 26:7), and God did claim that Abraham would command and teach his children to keep the way of the Lord (Genesis 18:17-19)? What can be said of these things?
First of all, we know that God told Abraham to come to this land and he would give it to him. Abraham believed God and it was credited to him as righteousness (Genesis 15:6). No matter where Abraham roamed, others ruled the lands that were promised to him, yet he believed God. According to the text, Abraham came into hostile territory, at least according to his understanding (Genesis 20:11), and he feared for his life. Yet, not once does the text show that Abraham considered returning to Mesopotamia from where the Lord called him, showing that no matter what the apparent conditions, Abraham believed God. So, how did Abraham get into this predicament? Was he seeking an alliance with Abimelech? Did he volunteer to give Sarah to Abimelech in exchange for an alliance and freedom to live in the land he ruled? No, Abraham did none of these things. Abimelech was the aggressor in all the things that transpired between him and Abraham.
Abimelech came and took Sarah, showing he believed he was superior to Abraham and dictated policy to him. Who wouldn’t fear for one’s life when coming into a land like this? When Abimelech did come to Abraham to discuss the matter, Abraham was straightforward, telling the king Sarah was indeed his sister, but also his wife. The fact that he was not forthcoming about their being married at the beginning was due to Abraham’s fear of what the king might do to him to gain Sarah for himself (Genesis 20:10-13).
Abraham told the truth, but not the whole truth. Did this make him guilty of wrongdoing? If this were true, what should we say about the Hebrew midwives who not only disobeyed Pharaoh but lied to him about the male babies they saved alive and were blessed by God for doing so (Exodus 1:15-20)? How then should we understand the Lord’s own advice to Samuel who feared for his life when he was sent to anoint David king (1Samuel 16:1-5)? Isn’t this example very similar to what Abraham thought when he came into Abimelech’s kingdom (Genesis 20:11)?
Are we then to conclude it is alright for the children of God to lie? No, for no liar will enter into the Kingdom of God (cp. Revelation 21:10, 27). The commandment of God is that we would not bear false witness (Exodus 20:16) and the context is when required by authorities to divulge information about a certain matter. It has nothing to do with turning the innocent over to an evil power in order that they might be destroyed or tortured (as was the case in Nazi Germany). It has nothing to do with being tactful when a mother asks a doctor if her son suffered before dying, or embarrassing the poor or the ignorant because he or she didn’t wear the appropriate attire at a formal event. The fact is, deception and lies may be required in certain and difficult circumstances to protect the innocent, as was required in war with Nazi Germany, and is required under any similar circumstance today when authorities don’t protect the innocent. Moreover, the whole truth isn’t necessarily a matter that concerns certain people, for why should we pave the way for them to do us harm (cp. 2Kings 20:13, 16-18). When the righteousness of God or the truth of God is under attack, deception may be necessary; but under normal circumstances where we are called upon by a legitimate authority (church, state, employer etc.) to witness to a matter (no matter how uncomfortable that may be) we are required by God to tell the truth.
 See Barnes Notes; Jamieson Fausset and Brown; Keil & Delitzsch; Believers Bible Commentary (MacDonald & Farstad) and others.