What is the subject of Mr. Hooper’s sermon on the first day he wears the veil?
In Nathaniel Hawthorne's short story "The Minister's Black Veil", the topic of the sermon is "secret sin". The story describes it like this:
...[SECRET SIN] and those sad mysteries which we hide from our nearest and dearest, and would fain conceal from our own consciousness..
It is clear that Reverend Hooper has undergone some traumatic or shocking event that has made him make the decision of covering his face from everyone forever using the gaudy veil. The hint of what happened may likely have to do with the death of a young woman from the community. Her funeral, which occurred in the afternoon of the speech, brought with it much commentary from the villagers. They made the conclusion that the woman and the minister had been lovers in secret, his secret sin, and that he is ashamed and pained by the death in such a way that he has decided to hide from society.
A superstitious old woman attending the funeral thinks the corpse shudders when the minister is near. Another woman imagines the minister and the spirit of the young woman walking together, holding hands in the funeral procession and, thus, linked in secret sin.
Therefore, it is very likely that the minister had been quite literal and not allegorical when he was talking about hiding his own sin from the world.
The topic of the minister's first sermon after donning the veil is "secret sin." This first sermon is overtly ironic, given the fact that that the new minister has chosen to randomly take up the habit of hiding his visage behind a veil. However, the delivery of the sermon is also paradoxical, since the minister is normally mild-mannered and not very exciting from the pulpit, but with the addition of the veil, the parishoners now hang on his every word--which is precisely the effect he hopes for.