You've probably heard the common accusation that Christians believe a myth, that the "Jesus of Easter faith" (with its Virgin Birth, miracles, resurrection, and ascension) is the stuff of legend and does not belong to the portrait of the historical Jesus. This accusation is intended to negate Christian truth claims because they are mythical. By labeling the core Christian message regarding the Incarnation as myth (thus exposing the superstitious and fraudulent nature of Christian belief), gainsayers look to drive moderns to their right mind, that is, into the mindset of agnosticism, deism, or atheism which, as the thinking goes, totally repudiate myths and other such folklore, fantasies, and fairytales.
C.S. Lewis was once challenged by his friend Cornelius on the connection between Christian beliefs about Jesus and myths that seemed to expose such beliefs as the stuff of legend. Lewis pondered that if Cornelius is right and there are mythical elements in the Gospel accounts, then why would Christians still insist on teaching the biblical narrative about Jesus "in terms of an archaic mythology which must hamper and embarrass them at every turn?" Lewis further wondered, "Why do they refuse to cut the umbilical cord which binds the living and flourishing child [i.e., the Church community and the social good it does] to its moribund mother [i.e., the myths of the Gospels that undermine it]?"  One would have thought that modern Christians would have abandoned this embarrassment and got on with the ethical teaching of Jesus without the Virgin Birth, atonement, resurrection, and ascension.
By myth, Lewis means a story that purports in some sense to be historical and which encapsulates and reinforces the strongly held beliefs of the community that tells it.
But Lewis came to recognize, like many before him, that Christianity can not be understood apart from its mythical framework and that myth is an important form of communication through symbols. In other words, rather than being a defeater argument to dismiss the Incarnation as a core Christian belief, mythology reinforces its legitimacy.
By myth, Lewis means a story that purports in some sense to be historical and which encapsulates and reinforces the strongly held beliefs of the community that tells it. Fairytales do not purport to be historical and, therefore, are not properly mythic. It is simply make-pretend storytelling. Serious myths, on the other hand, are regularly expressed not only in narrative but also in symbol and action. Symbols and action are powerful and meaningful ways to convey the subject narrated. For example, much of how we understand, interpret, and engage the United States is filtered through "the myth of America." It includes ideas like American Exceptionalism melded with the Pilgrim's historical and legendary "errand in the wilderness" to build "a city on a hill."  It includes mythical figures like George Washington, Ben Franklin, and Thomas Jefferson, who are now larger-than-life figures in American culture and identity, notwithstanding recent iconoclasm. But the mindset of Americans that comes from the "myth of America" wouldn't endure the rigors of hard-edged historical scrutiny. If the mythical aura is erased, then there would be nothing left but a colorless, meaningless voter-state. In part, this accounts for why the icons, the myths of America, are attacked and defended. Understand that the mythic element is inseparable from the historical reality (the actual persons, the confirmed events), and you have what was really and truly extraordinary now infused with meaning and significance. Lewis explains: "In the enjoyment of a great myth, we come nearest to experiencing as concrete what can otherwise be understood only as an abstraction" (God in the Dock, 66).
This is a crucial point. Myth, like poetry, may better convey reality than abstractions or factoids. Through myths, truth becomes dynamic.
In thinking about this, it's natural to ask, "Is Christianity then a myth like Gilgamesh or Jason and the Argonauts or Inanna and Dumuzi?" But that question misses the most important point: While Christianity certainly follows the "age of the myths of the gods" in terms of chronology, it does something totally different by importing symbols and actions into something that is truly historical. Hercules did not exist. Jesus really did. But the power of Hercules (as the destroyer of Hydra, etc.) was legendarily meaningful and augments the context and content of the powerful things Jesus did and represented in his person. Mythology infuses the historical verity of the Incarnation with a dimension immediately recognizable to those of Biblical faith.
The Biblical text resists binary reading, too.
"Yes," says the modern person, "But did it all happen or did it not?" Today, we want "just the facts" without reference to cultural allusions, metaphors, locations, contexts, characters, and actions. Did it all happen or not?
The idea of myth upends that kind of binary thinking. The Biblical text resists binary reading, too. Theology and history cannot be separated in the Gospels. The Gospels import the power, symbol, and meaning of myths into what is reported as historical facts concerning a real historical figure, Jesus of Nazareth. Jews recognized the miraculous. Gentiles discerned the legendary. Lewis offers his clearest thoughts here:
Now, as myth transcends thought, Incarnation transcends myth. The heart of Christianity is a myth, which is also a fact. The old myth of the dying god, without ceasing to be myth, comes down from the heaven of legend and imagination to the earth of history. It happens — at a particular date, in a particular place, followed by definable historical consequences. We pass from a Balder or an Osiris, dying nobody knows when or where, to a historical Person crucified (it is all in order) under Pontius Pilate. By becoming fact, it does not cease to be a myth: that is the miracle. 
Lewis restates what Christians have believed, taught, and confessed: the Christian assents to the historical fact and also receives the myth (though it has become fact). What became fact in Christianity previously was a myth in other cultures. Jesus Christ carries with him into the world of historical fact all the properties of a myth. Christ is more than Isis or Ra, not less. By extension, the God disclosed by Jesus is more than Zeus, not less. And it is here that Lewis makes his strongest point:
We must not be ashamed of the mythical radiance resting on our theology. We must not be nervous about 'parallels' and 'Pagan Christs': they ought to be there — it would be a stumbling block if they were not. We must not, in false spirituality, withhold our imaginative welcome. 
What points of contact would there be with first-century auditors, both Jew and Gentile, if pre-existing myths did not resonate with the Christian proclamation of the historical Jesus? Moreover, the imagination of people for thousands of years to this very day with superheroes and sky-gods seems to anticipate something along the lines of what the apostles reported, except their report was in keeping with the long history and narrative of Israel. Nothing could be more miraculous and yet expected, extraordinary and commonplace. If Christianity was not reporting something that smacked of the things of myth, then it could hardly be that which occurred "in the fullness of time." John would not be pleading with us to believe that "these things were written so that you may believe that Jesus is the Christ, the Son of God, and believing have life in his name." Christianity is not going for a corner of your heart or to be motivational in your life. The foundational claim of Christianity is that Jesus is the Lord of heaven and earth, life and death, the living and the dead; he is the Alpha and the Omega. In other words, it is going for the entire lot — from creation to the end of the world, and only myths carry that kind of cultural import. Christianity grounds myths in the real — the fact of Jesus and his redeeming life. Jesus is the miracle, and the myth melded to time, space, people, and place.
There is no reason, then, for a Christian to recoil when our beliefs about Jesus are associated with myths properly understood. When challenged with this charge, our answer should be, "Naturally. Did you expect anything less?" Christianity does not offer anything less or even the same. It offers so much more because it grounds the story of God in Christ reconciling the world to himself not "a long, long time ago in a galaxy far, far away," but rather over there (Judea), among them (the Jews and Romans), at that time (the reign of Tiberius Caesar), in this manner (representative life, atoning death, and resurrection) and with these implications (salvation for mankind and lordship over all). Theology and history go hand in hand in the real person of Jesus Christ, making the truth of the Gospels profoundly human and powerfully meaningful.
 C.S. Lewis, God in the Dock: Essays on Theology and Ethics (Grand Rapids: Eerdmans, 1970), 63.
 See Perry Miller, Errand in the Wilderness (Cambridge, MA: Belknap Press, 1956).
 Lewis, God in the Dock, 66-67.
 Lewis, God in the Dock, 67.