Sam Harris's bestseller "Letter to a Christian Nation" compresses into fewer than 100 pages pointed arguments that (1) religion in general and Christianity in particular are false in that they fail to prove claims of god(s), (2) they burden society and retard the pursuit of knowledge by fostering irrationality and immorality, and (3) with the increasing prospect that motivated religionists may employ modern warfare technology to press their views, they pose a danger to civilization. Douglas Wilson's "Letter from a Christian Citizen" offers a thoughtful response of roughly equal length.
Wilson says little about the first point, arguing mainly that the Bible's accounts of historical events/miracles serve as sufficient evidence. Those taking to heart Carl Sagan's admonition that extraordinary claims require extraordinary evidence will likely find this (well worn) argument wanting.
Wilson offers an interesting counter to Harris's observation that Christians understand "what it is like to be an atheist with respect to the beliefs of Muslims," since "[i]sn't it obvious that Muslims are fooling themselves?" As Harris puts it: "Understand that the way you [i.e., Christians] view Islam is the way devout Muslims view Christianity. And it is the way I view all religions." Wilson argues that this is a false analogy, since both Christians and Muslims at least understand that some god created the universe while atheists don't. He doesn't explain why it matters that Christians disbelieve both what Muslims and atheists think, but for different reasons since they think different things. Wilson seemingly acknowledges Harris's point later in the book, stating that "I believe that Islam is a false religion, and I believe that the people who adhere to it are deluded."
Wilson focuses most of his attention on Harris's attacks on the morality of Christianity. Rather than directly defend and explain the morality of Christian dogma, he mainly disputes whether or how an atheist can question the morality of anything, arguing that without reference to the Bible, no standards exist by which to judge the morality of anything. To the extent that Harris displays some sense of morality, Wilson argues, it is "a hodge-podge of Christian leftovers." Harris, on the other hand, argues in his book that it is the other way around--Christianity didn't invent morality and instead borrowed from an innate sense of morality common to humankind, adhering to it in some particulars and deviating from it in others.
With respect to Harris's third point, Wilson passes it off as largely a problem only with the "false religion" of Islam.
Wilson presents a variety of arguments (only a few of which are mentioned in the foregoing summary), and while I found them unconvincing, he presents them clearly and generally offers explanations, examples, and/or references to support them. He plainly displays considerable knowledge of his subject matter. I read "Letter from a Christian Citizen" out of curiosity about what responses could be offered to Harris's various arguments. While others undoubtedly can and will expand on those advanced by Wilson, his book offers a good sample of the thinking of some unconvinced by Harris.