What is Lewis saying about the relationship between good and evil?
Our first hint toward the answer to this question is the title of the work itself--The Great Divorce. C.S. Lewis chose this title in response to William Blake's play entitiled The Marriage of Heaven and Hell. Lewis' "divorce" title proclaims loud and clear his feelings about these two spiritual realms that embody good and evil; that there is absolutely no overlap. They are separate and distinct, not part of any union. As quoted in the study guide to the work provided by the C.S. Lewis Foundation (second link below), the title page of the work confirms this idea through the voice of George MacDonald: "No, there is no escape. There is no heaven with a little of hell in it – no plan to retain this or that of the devil in our hearts or our pockets. Out Satan must go, every hair and feather". This is particularly important considering the huge influence that MacDonald's writings had on Lewis' spiritual life and conversion, which you can read more about in the enotes C.S. Lewis topic page (first link below).Lewis is sending a clear message that he wholeheartedly believes in--there's no room for relativism there. There is heaven. And there is hell. There is good. And there is evil.
Looking beyond the title alone, we find in The Great Divorcea beautiful allegory that draws on many major texts that came before it, including ideas from Dante's Inferno. The landscape of The Great Divorce shows us Lewis' own vision of the afterlife, and makes clear exactly how he feels human beings may be categorized eternally in heaven or hell. Here, the relationship between good and evil shows itself to be not only irrevocably separate but also highly contingent on human choice and human conviction. Instead of the often seen idea of good or evil as overpowering forces that can consume us, Lewis gives humans all the power, which makes the fall to evil (and therefore Hell) even more of a tragedy. The bus passengers on their way to heaven must make a conscious choice to board, and once they realize all that is involved in gaining admittance to eternal paradise, they must repent completely with their full hearts. The feeble excuses given by many illustrate the fallibility of human nature and further reinforce the idea that the one connection between good and evil is our own ability to determine which of the two will characterize our eternal destiny.
Lewis is categorized as many things, being such a prolific writer.The Great Divorce falls under his work as a Christian apologist (see enotes link below), where he crafts both narratives and essays to create an argument in favor of Christianity. You'll likely find much more about the Lewis' view on the relationship of good and evil in his other apologist works, such as The Screwtape Letters, as well as any outisde literary criticism of Lewis' writings.