One of my favorite books by Abp. Fulton Sheen is Philosophy of Religion: The Impact of Modern Knowledge on Religion, originally published in 1948 by Appleton-Century-Crofts (New York). What follows is a short overview, originally published in This Rock magazine, of some points made by Sheen about comparing Christianity to other religions:
Archbishop Fulton J. Sheen outlined the problems with the arguments that Christianity was derived from pre-Christian paganism, made in the early twentieth-century by men such as H. G. Wells, H. L. Mencken, and Sir James G. Frazer (author of The Golden Bough, an influential study in comparative folklore, magic, and religion). In Philosophy of Religion: The Impact of Modern Knowledge on Religion, written nearly sixty years ago and aimed at more scholarly works, Sheen lists false assumptions underlying comparative religion that provide a helpful apologetic yardstick for gauging works that claim "all religions are the same" or that "Christianity stole its beliefs from pagan religions."
First false assumption: Religion represents the primitive instincts of man and the "infant stage" of civilization and "progress." Thus, the religion of primitive peoples today (say, the aborigines) represents the religion of ancient man. The mistake here is conjoining "the lowest forms of humanity with the oldest." This is the mythology of inevitable human progress, which assumes that the spiritual insights of the twenty-first-century man must be of a higher order than those of ancient nomadic Hebrews or third-century Christians. It underlies the assumption that either science or "spirituality" (non-Christian, of course) supplants religion as man evolves into a higher state of intelligence, "consciousness," or humanity.
Second false assumption: The true religion must be completely different from all other human religions. Since Christianity and non-Christian religions are not completely different in every detail, "it is falsely concluded that the Christian religion is not divine." Christianity and other religions do resemble each other in certain ways when it comes to natural truths, since those truths "may be known to anyone endowed with reason." These include moral teachings, use of symbols, and certain liturgical themes. Even when it comes to supernatural truths, Christians and pagans still share some general ground, such as the awareness of sin and the need for a redeemer. The key difference is that Christians "know definitely that Redeemer is Christ." The distinctively Catholic doctrine is that God does not ignore or discount our human nature but perfects it through supernatural grace.
Third false assumption: Resemblances between Christianity and other religions are "possible only through plagiarism, or borrowing, or imitation." A distinction must be made between complete and partial borrowing. If the borrowing is complete, then Christianity is not unique nor divine. But "there are some elements in Christianity that are absolutely original," despite some wild and unfounded claims: the historical facts of Christianity and the unique doctrines of the Catholic Church, including the Trinity, the Incarnation, and the Eucharist.
Fourth false assumption: Since the supernatural is impossible or cannot be demonstrated, religions and the history of religions "should be studied apart from all intervention of God." But "it is one thing to abstract the supernatural character from religion and quite another thing to deny the supernatural." Since Christianity claims to be supernatural, it should be investigated as if it were so. Failing to consider this claim, upon which all of Christianity rests, reflects a materialist bias, often presented as "scientific."
Also read the accompanying longer article I wrote for the May-June 2006 issue of This Rock, titled, "Jesus and the Pagan Gods".