Mere Christianity is Lewis’s best known and most complete work of apologetics. In it he begins with a general argument for theism (the existence of God) and then expands that argument into a specific defense of the Christian gospel. From there, he goes on to explain and support the central moral and theological principles of Christianity.
Although Lewis believed firmly in the authority of scripture, he knew that many of his modern readers did not share his belief. Accordingly, Lewis carefully builds his apologetical arguments on common ground: on facts and observations about our world and ourselves that all people, regardless of their religious beliefs, can see, understand, and acknowledge.
That is why he begins Mere Christianity with an unexpected statement that seems, on the surface, to have little to do with a defense of the Christian faith. Did you ever notice, Lewis writes, that when two people disagree about something, they argue about it rather than fight? Though most of us likely did not notice this phenomenon before, the moment we read Lewis’s statement, the truth of it becomes apparent. Of course we argue instead of fight!
And that’s when Lewis hooks us. Whether we realize it or not, two people cannot argue about something unless they agree (often unconsciously) to a fixed standard that transcends them both. When we argue, we take that standard for granted and then make a case (sometimes rationally, sometimes irrationally) that our side of the argument better approximates that standard.
In a case where two former business partners are suing each other for fraud, neither party says: “yes, I swindled my partner, and I was right to do so.” If he did, he would not be sent to jail; he would be sent to an asylum. Now, one party might partially confess to fraud, but then he would follow the confession by offering mitigating circumstances to show that the “fraud” was actually justified. In other words, he still holds to the accepted standard that fraud is wrong.
On the basis of our shared experience of such ethical debates, Lewis posits that a universal, cross-cultural moral code exists and is binding. In The Abolition of Man, he gives that law code a name: the Tao. Many Christians are confused by this: why should Lewis borrow a word from Taoism (a branch of Buddhism) to bolster his case for the Christian faith? The answer is simple: to show that all people (east and west) recognize the Tao, even though they continually break it.
Many relativists will balk against Lewis’s assertion of the Tao, claiming that morality veers wildly from culture to culture and is a man-made (rather than a divinely-given) thing that alters from age to age. But those same so-called relativists will quickly change their tune if someone robs them. “It was wrong of you to do that,” they will say, and if the person who robbed them says, “in my culture it is OK for me to steal,” the relativist will not accept the excuse.
The fact is everyone knows the Tao exists, for whatever our own personal ideology, we expect other people to treat us in accordance with the Tao. Indeed, if there were no Tao, then no court could have tried the Nazis or Saddam Hussein or the perpetrators of apartheid. The Tao does exist, but if it exists, then it makes necessary a director of the Tao who transcends all times and cultures. It requires, in short, a super-natural Creator who inscribed the Tao into our conscience.