In his late teens, C. S. Lewis was an avid atheist, and considered Christianity merely one of many mythological belief systems. His mother's early death fueled his rejection of Christianity: he could not believe that a God who allowed such suffering could be "good". Furthermore, he considered "belief" to be a form of bondage.
Lewis's opinion on religion changed dramatically in 1931, when he underwent a conversion. As an undergraduate student of philosophy, he firmly believed in the existence of an "absolute" morality, but his conversion combined his notion of absolute good with the Christian God. The conversion had been prompted by long talks with his friends, and was finally sparked by his brother Warren's own conversion. This new perspective informed Lewis's later writings, and perhaps most notably The Chronicles of Narnia. Lewis came to believe that the story of Christianity was a "true myth" - a myth that actually happened.
In 1940, Lewis wrote a book for a Christian series published by a small firm called Centenary Press, led by Mr. Ashley Sampson. Lewis was asked to write a book called The Problem of Pain (1940), which became his first work written from the stance of Christian apologetics. The book was particularly successful due to the clarity of its language and thinking. It addressed the question of why, if God is good, he would allow suffering to exist in the world. Beginning with an explication of the origin of religion and how earthly activities came to be attributed to a powerful and good "creator", Lewis argued that pain and suffering exist, together with free will, to compel a person to change for the better, via the positive effects of pain. In other words, Lewis defined and defended the Christian doctrine that one is made "perfect" through suffering.
Lewis then collaborated with the BBC on a radio series that further distilled his thinking. The BBC approached him with two platforms: (1) to lead a discussion on modern literature and Christianity, or (2) to discuss Christianity from a layman's point of view. Lewis opted for the latter, claiming that he did not read much modern literature anyway, nor did he feel at ease with its attitude toward faith. The BBC radio series, which aired live, proved a success, in particular because of Lewis's clarity and intelligence, as well as the apt responses he gave to listeners' questions live. In 1952, Mere Christianity appeared in its final book form, and Lewis became a popular public figure, frequently speaking on issues of the Christian faith.
Although Lewis was not a clergyman, his status as a former atheist and scholar gave him a unique perspective that spoke to other laypersons. He was able to connect with ordinary people, and especially skeptics. He avoided specific discussions about the Church and its varied conflicts concerning dogma, and concentrated instead on the basic core values shared by all Christians. It was from this position that he embarked on The Chronicles of Narnia, which he used as a literary vehicle to teach the values that he discussed in his other works.