In The Scarlet Letter by Nathaniel Hawthorne, how is the phrase "Be true" a central theme of the story?

A lifelong preoccupation of Hawthorne was the hypocrisy of
his Puritan ancestors and the resulting guilt that he felt for their sanctimonious
transgressions. Of all sins, the sin of hypocrisy is grave, for while hiding guilt, it
actually compounds guilt.  After all, the more guilty one is, the more he feels the need
to lie in order to hide his secret sin. Then, he feels guiltier; and, if he does wish to
confess, no one believes him.  Either way, psychological anguish
results.

As an example of the benefits of truth being
exposed, Hester's scarlet A, while causing her ostracism from the
Puritan community, does, at least, afford her the opportunity for honesty and for
spiritual growth.  While she suffers, Hester also develops into a caring and empathetic
listener and nurse of the spiritually and physically ill of her community.  So great is
her spiritual growth that Hester's letter comes to symbolize "Able" or "Angel."  On the
other hand, her secrecy about Roger Chillingworth's being her husband reaps great
anguish in Hester's heart until she finally discloses this truth to Arthur
Dimmesdale.

As the incarnation of Hester's and Arthur
Dimmesdale's sin, little Pearl cannot be fully human until Dimmesdale confesses.  Then,
Pearl no longer must be but a symbol of Hester's passionate nature; now she converts to
her humanity as she finally displays love and emotion as she kisses
Dimmesdale.

With no other character is the profound verity
of Hawthorne's exhortation to "Be true!" more evident than in the character of Reverend
Arthur Dimmesdale.  His hypocrisy causes him such mental and spiritual anguish that he
seeks to relieve it through physical pain in self-flagellation.  Continually, holding
his hand over his troubled heart as he suffers from guilt and from the vengeful preying
upon his guilt by the nefarious Roger Chillingworth.  When Dimmesdale confesses his
"vileness" as a sinner, the community misinterprets his honesty by thinking him a saint
who humbles himself.

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"He had told his bearers that he was altogether
vile, a viler companion of the vilest, the warts of sinners, a thing of unimaginable
iniquity. . . . They heard it all, and did but reverence him the
more."

And, so, Dimmesdale's
guilt is compounded so much that it manifests itself in his stigmata upon his chest, and
he is eventually killed by his terrible anguish.

Likewise,
Chillingworth, whose loathsome sin of vengeance causes him to sink into spiritual
depravity, thus appearing craven and evil, is deceptive and hypocritical.  His devilish
business turns Chillingworth himself into a fiend.  And, when he is rejected by
Dimmesdale, Chillingworth "withers like an uprooted weed left in the sun."  But, if
Chillingworth had been honest with Hester Prynne from the beginning, he should not have
married her; he sinned against Nature as he knew that he could provide what she
needed:

It
seemed a fouler offence committed by Roger Chilllingworth, than any which had since been
done him, that...he had persuaded her to fancy herself happy by his
side.

His most grievous sin
is what Hawthorne terms the "unpardonable sin," the subordination of the heart to the
intellect. Chillingworth sacrifices Dimmesdale to the gratification of his own selfish
interest.  But, his vengeful "violation of the human heart" turns upon Chillingworth and
destroys him: In trying to play God, Roger Chillingworth becomes a fiend. After
Dimmesdale's confession, he withers and dies.