Religion serves a unitive purpose. In uniting the person to God, religion unites people together. However, many religious people forget religion's purpose. They like to puff up their egos, reveling in their ability to live according to the Law. Seeing themselves as better than the rest, they forget that grace only comes to those who realize they are sinners. The tax collector, realizing he is a sinful man, does not focus on himself, but focuses his gaze and hunger on God - the source of salvation. Justification comes to those who do likewise.
