Myths as a basis for believing in Christ

One of the things that many detractors from Christianity love to throw out there is the similarity between things from the Scriptures and other ancient religions that may have been older than what happened in the Scriptures. Recently, I've seen lots of posts about Horus or Osiris and how their stories both predates Christ's and are very similar. I've always wondered wether this is a valid criticism of Christianity, maybe they are correct in assuming that Christianity has just borrowed concepts from older religions. But, being a believer, I've always thought that there had to be another explanation.

I've wondered if the reason as to why many of the Biblical stories mirror other myths is that God has used those myths as a way of preparing the way for all peoples to have some sort of base so that they can more easily accept the truth of God. Lewis seems to think something similar as he notes in Is Theology Poetry?  

He notes that theology offers special revelation, but also a general one too. "We should, therefore, expect to find in the imagination of great Pagan teachers and myth makers some glimpse of that theme which we believe to be the very plot of the whole cosmic story - the theme of incarnation, death and rebirth. And the differences between the Pagan Christs (Balder, Osiris, etc.) and the Christ Himself is much what we should expect to find. The Pagan stories are all about someone dying and rising, either every year, or else nobody knows where and nobody knows when" (Lewis from Is Theology of Poetry?). Lewis points out that comparing the myths to Christianity is "like watching something come gradually into focus; first it hangs in the clouds of myth and ritual, vast and vague, then it condenses, grows hard and in a sense small, as a historical event in first century Palestine".

When Christ came down to earth he partly emptied himself of his glory. Lewis points out that all things come down from heaven to earth (general revelation) and in doing so they too have been emptied partially of some of their glory. That is why myths are partially correct. In and of themselves, the myths can't lead to salvation; but they can provide the basis for coming to understand the truth that leads to salvation.

I'd love to explore this topic further.