In the interest of truth, I should first disclose the fact that Christian theologians are divided on this subject. Some—like Saint Augustine—believed that it is never permissible to lie. Others—like Dietrich Bonhoeffer, who had ample time to contemplate this issue from the perspective of a Nazi prison cell—held that under certain circumstances lying was not only morally permissible but morally mandated. Thus, Bonhoeffer advocated deceiving the enemy in circumstances of war, and he had no compunction about lying in order to facilitate escape for Jews facing extermination.
Furthermore, while the Bible never condones lying qua lying (lying for the sake of lying), it does condone lying in order to preserve a higher moral imperative. For example, Rahab purposed to deceive (the lesser moral law) in order to preserve the lives of two Jewish spies (the higher moral law). Likewise, a Christian father today should not hesitate to lie in order to protect his wife and daughters from the imminent threat of rape or murder.
Finally, there is a difference between lying and not telling the truth. This is not merely a matter of semantics; it is a matter of substance. By way of analogy, there is a difference between unjustified and justified homicide. Murder is unjustified homicide and is always wrong. Not every instance of killing a person, however, is murder. Capital punishment and self-defense occasion justified homicide. Similarly, in the case of a lie (Annanias and Sapphira, Acts 5) there is an unjustified discrepancy between what you believe and what you say, and so lying is always wrong. But not telling the truth in order to preserve a higher moral law (Rahab, Joshua 2) may well be the right thing to do and thus is not actually a lie.
For further study, see Norman L. Geisler, Christian Ethics: Options and Issues (Grand Rapids: Baker Book House, 1989), chapter 7.
“The king of Jericho sent this message to Rahab: ‘Bring out the men who came to you and entered your house, because they have come to spy out the whole land.’ But the woman had taken the two men and hidden them. She said, ‘Yes, the men came to me, but I did not know where they had come from. At dusk, when it was time to close the city gate, the men left. I don’t know which way they went. Go after them quickly. You may catch up with them.’ (But she had taken them up to the roof and hidden them under the stalks of flax she had laid out on the roof.)”