It is a part of our nature to survive. Faith is an instinctive response to aspects of existence that we cannot explain by any other means, be it the moral void we perceive in the universe, the certainty of death, the mystery of the origin of things, the meaning of our lives, or the absence of meaning. These are basic and extremely simple aspects of existence, but our limitations prevent us from responding in an unequivocal way and for that reason we generate an emotional response, as a defense mechanism. It's pure biology.