The subject of Mr. Hooper's sermon on the day he first wears the veil was preached deliberately to cause the parishioners to reflect upon their own actions and doings. Hawthorne writes that, whether it was real or a product of the imagination of the people, the sermon
was tinged, rather more darkly than usual, with the gentle gloom of Mr. Hooper's temperament.
That, combined with the enigmatic (and creepy) thick, black veil covering his face throughout the sermon, surely must have helped to accentuate any discourse ever to be given. This particular sermon went hand in hand with the tone used by Hooper, as well as with the prop of the veil: It was about secret sins.
The subject had reference to secret sin, and those sad mysteries which we hide from our nearest and dearest, and would fain conceal from our own consciousness, even forgetting that the Omniscient can detect them.
Clearly, the veil is an illustration of the speech. It hid its face the same way that all the parishioners presumably hide their secret sins behind the façade of righteousness and socially-acceptable actions that compose their social personas. The narrator goes on to say how the speech about secret sins seems to have touched a nerve in the audience members, who grasped their hands and clutched them against their chests in what the author calls an “unsought pathos” that seemed to suddenly take over the entire flock. This is a reflection of the state of the people’s consciences and how they quietly agreed with what the minister said.