We could do with a fuller understanding of what it means to say that Christ and the Christian God, among others, are mythological beings.
Let’s start with the New Testament, which contains many of the documents of early Christianity. (There are others--more about that later.)
Now let’s distinguish among three scholarly approaches to these documents: (1) the theological, (2) the historical, and (3) the comparative. The theological way is from the perspective of the believer, the historical way from the perspective of the historian, and the comparative way from the perspective of comparative religion.
We’ll call the theological way an “inside” perspective, because a theologian is a player in the Christian game; the other two are “outside” perspectives. This doesn't imply that either the historian or the comparativist can’t also be believers, just as the historian might also use comparative methods, or vice versa.
Christian theologians are believers who investigate the New Testament in the light of some creed (for instance, the Apostle’s Creed) or theological system (say, that of Thomas Aquinas). They might subscribe to the idea that Scripture is inerrant; or that when the Pope speaks ex cathedra (Latin, “from the chair”), his interpretation of church doctrine is correct and fixed. Moreover, they might interpret Scripture either literally or as consisting of a mix of literal and symbolic material. They might also use the historical and/or comparative methods. But the point is that a Christian theologian views the writings included in the Bible with the eyes of faith.
Theologians can be either conservative or liberal, or somewhere in between. Basic conservatives will argue that the Bible in inerrant and therefore authoritative, citing verses such as I Timothy 3:16: “All Scripture is given by inspiration of God,” or II Peter 1:21: “Holy men spake as they were moved by the Holy Ghost.” (King James Version) In other words, the conservative theologian will cite Scripture to prove its inerrancy, thus committing the logical fallacy of petitio principii, or “begging the question”--that is, assuming the truth of the conclusion in arguing for that conclusion.
Liberal theologians, on the other hand, will accept the findings of Historical Criticism (for example, by accepting a historian’s finding that Jesus never thought of himself as the Christ) yet find ways to maintain their faith by adhering to some theme of Christianity, whether it be certain teachings of Jesus, forgiveness, or those parts of Scripture that speak of peace and justice.
Whatever form it takes, the inside perspective runs the risk of either ignoring or being ignorant of the historical and comparative methods and their results. Christian theologians can certainly incorporate these two “outside” perspectives into their work, though if they are to consider all available evidence, they must be able to “bracket” their convictions, at least temporarily. Put differently, they must learn to view the evidence as if they were uncommitted outsiders.