Many people think that the purpose of this book is to convince atheists to become Christians. While it has been effective in this regard and while the arguments are inspired by Lewis’s own conversion, his main goal is to explain Christianity (apologesis, after all, means “explanation”). Lewis is fully aware that Christianity cannot be proven and involves some amount of faith, but he notes atheism also requires a great deal of faith. His main goal is to show that Christianity is reasonable. It is not a random superstition or popular myth, but a complex system of thought derived from logical processes and purported historical evidence.
Therefore, like a Pauline epistle, Mere Christianity is built on the core claim that God became man in Jesus Christ then died and rose from the dead to allow for humans to participate in the divine life of the blessed Trinity. In this context, moral behavior is seen as the way to better participate in that divine life.
Interestingly, while most Christian concepts of the Trinity emphasize the relationship of love, Lewis seems to emphasize a relationship of power or being. The Father shares power and wisdom with the Son through the Holy Spirit. The Christian, by the indwelling of the Holy Spirit and the mediation of Christ, can participate in the Father’s power and wisdom through prayer. Morality is in that sense a means of self-discipline to handle God’s power properly.
Lewis’s view of salvation is derived from Plato’s myth of the cave, a recurring concept in both Lewis’s fiction and nonfiction. Heaven is a state of reality that exists beyond people’s comprehension. The goal of a spiritual life is to prepare the soul to face the raw power of that reality when the time comes. Christianity is unique because it is the only religion that has God stooping down to the human level to lift people up.