Clive Staples Lewis (1898-1963) enjoyed a distinguished career at Oxford and Cambridge. He was also a notable literary critic and author of science fiction and children's literature (including the Chronicles of Narnia). In addition, Lewis was arguably the most influential Christian apologist of the twentieth century. Remarkably, he was a committed atheist before his conversion to Christ in 1929.
Lewis authored a number of important apologetic works, such as Miracles, The Problem of Pain, God in the Dock, and The Abolition of Man. In his most famous work, Mere Christianity, Lewis presented powerful arguments for the truth of the Christian faith. Originally broadcast as several BBC talks during World War II, Mere Christianity notes that even people who deny objective right and wrong cannot refrain from believing in them. Moreover, people are unable to live out the moral law they know they should. Lewis argued that this moral law, coupled with humanity's inability to fulfill it, allows Christianity to begin to "talk." The forgiveness God offers in Christ makes sense in the real world.
Lewis also maintained that Jesus Christ claimed to be God, undercutting popular notions that Jesus was something like a good teacher. Either He was who He claimed, or else He was a liar or lunatic. But the life of Jesus does not betray the character of a liar or the mentality of a lunatic. Lewis contended that the most reasonable understanding of Jesus is that He is the Lord.
Extracted from the Apologetics Study Bible.