How would you sum up C.S. Lewis' argument in "The Inner Ring" in one or two sentences? Where does he state his argument directly? Can his argument apply to life in Christian circles as well as the broader society?

Pinning Lewis's argument down in two sentences would likely oversimplify his argument, but he does state clear objectives. First, he says, “My main purpose in this address is simply to convince you that this desire [to enter an inner ring] is one of the great permanent mainsprings of human action.” Lewis wants to make sure we're aware of this innate drive. He continues his address, which he's stated is a warning, to point out the way that the pursuit of inner-ring status—being inside a privileged, special group—is not only natural, but destructive, so each of us should guard against it. The path to related “scoundrelism” appears in “in no very dramatic colours,” so it can be a challenge for us to recognize our drift in that direction, prompting Lewis's concern.
In secular life, we see social media encouraging new forms of these circles of division. Alan Jacobs writes well on the topic in his recent book How to Think. The “inner ring” idea does apply to Christian circles; any way in which the church or a community of believers relies on humans, it's susceptible to human fallibility. It would be worth thinking through the ways Christian communities you've seen may have inner rings, whether that be in leadership or through cliques. New Testament descriptions of ideal communities (open to interpretation, with modern scholars not in agreement about exactly what these ideals are) show ways in which this human tendency can be countered.