What is the topic of the first sermon Mr. Hooper gives while wearing the veil in "The Minister's Black Veil"?

The topic of the Reverend Mr. Hooper's sermon is secret sin.

Since the Calvinists (such as the Reverend Jonathan Edwards) of the Puritan era are sin-obsessed, at the center of their theology is an uneasy mixture of certainty and doubt. Certainly, because of Adam and Eve's grievous sin of disobedience, most humanity would be damned for eternity. Of course, doubt centers on whether a particular individual is one of those who will be saved (the...

The topic of the Reverend Mr. Hooper's sermon is secret sin.

Since the Calvinists (such as the Reverend Jonathan Edwards) of the Puritan era are sin-obsessed, at the center of their theology is an uneasy mixture of certainty and doubt. Certainly, because of Adam and Eve's grievous sin of disobedience, most humanity would be damned for eternity. Of course, doubt centers on whether a particular individual is one of those who will be saved (the Elect), or one who will be damned (Unregenerate). Since the Puritans do not know to which group they belong, they all try to live an exemplary life; they also try to hide their sins lest they be thought to be among the Unregenerate.

It is within this anxious spiritual milieu that Mr. Hooper dons a black veil across the lower portion of his face, an act that causes immediate discomfiture and doubt on the part of his congregation. With the veil on his face, Mr. Hooper's sermon is linked personally to his topic. He suggests in this sermon that each person has secret sin, some shame that he or she wishes to conceal from others and even him/herself, forgetting, however, that the Almighty can detect any and all sins. 
In addition to this disturbing sermon, the symbolic and mysterious shame that the veil seems to communicate causes his audience to react personally, feeling that their own shameful veiled acts are threatened.  

After the service one of the women remarks, "How strange that a simple black veil...should become such a terrible thing on Mr. Hooper's face." And her husband observes that the veil, though it only covers the lower portion of Mr. Hooper's face, "throws its influence over his whole person, and makes him ghost-like from head to foot."
So disturbed are many in the congregation that they avoid the minister until he becomes alienated from all, even his own fiancee. Yet, he refuses to remove his veil, declaring even on his deathbed, "I look around me, and lo! on every visage a black veil!" implying that each man and woman hides something.