What is the main theme of "The Scarlet Letter"?

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 twolove and hateare 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 naturesnatures that we all, according to this narrator, possessthen it becomes easier
to admit it!  Everyone is in the same boat, so we can and should be
honest.