Fireproof shows identity to be driven by individuals, but needing assistance from an external reality. Essentially, Caleb is lost in the wilderness of self- pity, internet porn, and resentment until he finds the "Love Dare" program and the spiritual identity of born- again Christianity. When this external reality enters into his daily pattern of existence, true identity is revealed. This is the same with Catherine, who is immersed in her own self- destruction mixed in with carnal desire until she recognizes the truth of Caleb's spiritual resurrection. The renewal of their vows at the end of the film is reflective of how they have shed their past forlorn ways and entered into an identity that is more of a covenant with an external reality of religious notions of the good.
The ending with the Salt and Pepper shakes helps to illuminate how identity without the divine can result if self- destruction and wedding one's identity to something larger than the individual is the only way for happiness to be evident. This becomes one of the primary messages of how identity is constructed in the film.