In the final chapter, the narrator suggests that love and hate are, in many ways, the same. He says,
It is a curious subject of observation and inquiry, whether hatred and love be not the same thing at bottom. Each, in its utmost development, supposes a high degree of intimacy and heart-knowledge; each renders one individual dependent for the food of his affections and spiritual fife upon another: each leaves the passionate lover, or the no less passionate hater, forlorn and desolate by the withdrawal of his subject. Philosophically considered, therefore, the two passions seem essentially the same, except that one happens to be seen in a celestial radiance, and the other in a dusky and lurid glow.
In other words, both love and hate require a deep knowledge and understanding of the other person; both render the person who feels the intense emotion dependent upon the person for whom they feel it, and both feelings leave the lover or the hater without purpose if the object of their feelings is removed. Thus, the narrator reasons, the two—love and hate—are basically the same, only that love is seen as something divine and hatred is seen as something evil. This is another theme of the novel.
Moreover, as the narrator says in this same chapter, "in the view of Infinite Purity, we are sinners all alike." We are all sinners and pretending to the world that we are not would actually be another sin. We would be living a lie. This idea underwrites his command that we "Be true!": if we admit to our sinful natures—natures that we all, according to this narrator, possess—then it becomes easier to admit it! Everyone is in the same boat, so we can and should be honest.