The mystery of theism is a development within the essence of each person; a development which - guided by God, I believe - is both evolutionarily and spiritually beneficial. This development cannot be traced through empirical or rational inquiry; in fact, it is beyond the capacity of the human species to effectively trace even the contours of the search for that which lies beyond the phenomenal world. We can only respond to the desire for the divine, and speculate on the source of that desire.
Many theories may be posited to explain the search, but in the end it is simply a desire - a desire for transcendence that cannot be systematized or rationalized - one that is not only within every person but also beyond every person. It is both a human and a conscious trait, and humans are the only beings (as far as we know) who are aware of this conscious trait within themselves.
Everyone desires transcendence, whether they recognize it or not. Even the most committed materialist cannot escape the desire for transcendence. It is within every attempt to move beyond our limitations, within each surge of imagination, underneath every action that we claim is "new" or "progressive."
It is certainly not proper to call Christianity a fundamental philosophical notion. First, one must hold to theism, or Christianity becomes an exercise in folly. But, if theism is tenable in any fashion, then Christianity becomes a vaible form of theism for consideration.
In spite of the culturally relative aspect of religious development, one must decide to commit to a particular religious (or non-religious) faith at some point in one's life, if only because there is no way to avoid some commitment, given the freedom of will inherent within each person.
Of course, there is another side to this: God is drawing all those who will turn to God, and Christians believe that this is done through the person and work of Jesus Christ, who is God incarnate. However, this is the language of faith, the language of the committed, and as such, even if the work of God in Christ does precede all else, we can only speak of it subsequently to our religious development, therefore we can only view it as second - not secondary! - to the desire for transcendence.