The continuity amidst the various conceptions of the “law of nature” manifested across the globe is indeed crucial. It is perhaps the strongest practical testament to the existence of an externally (supernaturally) administered standard of morality. It is a simple, yet often overlooked concept. This evidence for the law of nature presents a major problem to any worldview or religion which denies the existence of an absolute standard.
Certain proponents of such worldviews (atheistic humanism for example) may say that each person does, in fact, have an instilled sense of morality, but that it is relative to each individual. No one person’s morals have any higher value than another’s. The similarities are merely coincidental, or simply because of the common factor of humanity.
That standpoint falls apart quickly, however, because those who take it, perhaps many times subconsciously, place a higher value on certain individual standards than on others. To provide a classic example: few humanists would attempt to justify the actions of Hitler, though he was acting upon his strong personal standard of morals and ethics. Thus, they place their own standard, and the standard of the majority of the world, at a higher value. They may call him an “exception,” but for there to an exception there must be a rule, and therein have they proved themselves wrong.
It is quite fitting that Lewis opened one of his most well-known books, Mere Christianity with this issue, as it truly lies at the foundations of Christianity, if not all of Philosophy and Theology. Likewise, I find it no wonder that so many other religions choose to deviate from the truth at such a fundamental level.
The use of subtlety such as Lewis employs in the reading can be a very shrewd and effective decision in presenting Christianity. Most people have some preconceived notions about Christianity that are not necessarily accurate or positive. As such, jumping right into “Christianity” talk can cause people to immediately turn their minds off to the conversation. When talking with non-believer friends, often the most “productive” conversations begin with basic philosophical questions that inevitably lead to God, eventually. Often people are probably more willing to talk about neutral philosophical topics than about Christianity.
This type of discussion is especially more desirable for an intellectual, reasonable thinker type of person. A discussion that doesn’t directly use the Bible but still follows its principles can be much less intimidating and more effective. Quoting scripture left and right may be fine for discussions among Christians, but non-believers do not hold the same value or faith on the Bible, and it is thus useless to use it with them. If I were engaged in philosophical conversation with a reasonable, non-Christian friend, I would most likely use this method. In fact, I have on multiple occasions conversed with friends in a very similar manner to how Lewis laid out his argument.
Most any issue can be boiled down to an issue of morality, and further to the origin of one’s standard of morality. It is a powerful argument, and one that can rely on human logic and reason alone. This allows the foundation to be laid long before any spiritual aspects are introduced.
I have noticed that Lewis often uses analogies to illustrate the concept he is trying to communicate. Though no analogy is perfect, they can be very effective when it comes to ideas that may be normally confusing, difficult, or just weird. I often think in terms of analogies, and have personally found them useful both when wrapping my head around things myself, and when explaining things to friends.
His tennis analogy serves to help explain how we are not morally perfect, and shall never be, but why we should strive for moral perfection nonetheless. There are some people who may argue that because we will never be perfect there is really no point in trying. Giving the example of a tennis player provides a relatable way to understand why that is nonsense. The more an athlete trains to hone his skill, the more effective he becomes. Just because he will never be 100% effective is no reason for him to give up trying his best to reach his highest potential. If he were to do that, he would be considered a very poor athlete, or else not one at all. As Christians, we too can train our “moral muscles” in order get as close to perfection as possible.
Simply putting forth the effort to doing the right thing as much as possible can help tremendously. Doing so will cause it to become easier and arrive more naturally over time, so that slipping up will be the exception instead of the rule, and all the more so as time goes on. Athletes prepare themselves in other ways besides practicing their sports. For example, they condition themselves mentally, and nourish themselves as well as possible. We can further prepare ourselves by nourishing ourselves in the Word and spending time in intimate prayer and meditation, communing in the presence of the spirit.