In The Scarlet Letter, what underlying meaning does Hawthorne explore by making Hester and Dimmesdale both alike and unlike each other?

The similarities and differences between Hester Prynne and
Arthur Dimmesdale of Hawthorne's The Scarlet Letter revolve around
the effects of their mutual sins.  For, while they are both guilty of adultery, the
effects of their sins upon them greatly differ; Hester's is an open sin, and
Dimmesdale's is a concealed sin.  And, it is in the examination of the
psychological effects of Dimmesdales's secret sin in contrast to Hester's declared sin
that Hawthorne explores in his novel.

Because hers is an
openly acknowledged sin, Hester initially suffers horribly from the scorn of the Puritan
community; however, she maintains her pride and boldly displays her scarlet letter with
golden embroidery and elaborateness.  Likewise, she dresses Pearl, her living
conscience, in scarlet.  For Hester, her salvation lies in truth.  In Chapter XVII, "The
Pastor and His Parishioner," after Hester confesses that Chillingworth is her husband,
she tells Dimmesdale,

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In all things else, I have striven to be true! 
Truth was the one virtue which I might have held fast, and did hold fast, through all
extremity; save when they good,--thy life,--thy fame,--were put in question!  Then I
consented to a deception.  But a lie is never good, even though death threaten on the
other side!

But, unlike
Hester, Dimmesdale is weak, and is unable to confess his sin for seven years.  During
their forest conversation, he tells Hester,

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"...thou little, little knowest all the horror of
this thing!  And the shame!--the indelicacy!--the horrible ugliness of this exposure of
a sick and guilty
heart..."

And, so, Dimmesdale
suffers the torment of his conscience in the cowardice of his nature as he attempts to
rationalize that he is serving God by keeping his secret in order to continue to preach
and serve the community.  But, his fight is really against his own conscience--a fight
to which he loses his life, while Hester who is open about her sin mainains her strength
and finds redemption for her sin.  Thus Hawthorne's underlying meaning in exploring the
psychological effects of secret sin are given meaning in Dimmesdale's death and Hester's
survival.  When Dimmesdale finally has his victory of soul after the Election Day
sermon, Chillingworth's reaction demonstrates the significance of Dimmesdale's
confession and truthfulness.  He tells Dimmesdale, "Hadst thou sought the whole earth
over, there was no place so secret,--...where thou couldst have escaped me,--save on
this very scaffold! In fact, Hawthorne himself underscores the importance of confession,
a principle contrary to that of Puritanism that forbade sin of any kind. In Chapter
XXIV, "The Conclusion," Hawthorne specifically points out his meaning suggested by the
differences in Hester's open sin and Dimmesdale's secret
sin:

Among
many morals which press upon us from the poor minister's miserable experience, we put
only this into a sentence:--"Be true!  Be true!  Be true!  Show freely to the world, if
not your worst, yet some trait wherby the worst may be
inferred!"