What is the topic of Father Hooper's first sermon? Why is this important? "The Minister's Black Veil"

The topic of Father Hooper's sermon was hidden sin..... important because this was the first time he had withdrawn behind the black veil that would forever hide him from his congregation.

The sermon which he now delivered was marked by the same characteristics of style and manner as the general series of his pulpit oratory, but there was something either in the sentiment of the discourse itself or in the imagination of the auditors which made it greatly the most powerful effort that they had ever heard from their pastor's lips. It was tinged rather more darkly than usual with the gentle gloom of Mr. Hooper's temperament. 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. A subtle power was breathed into his words. Each member of the congregation, the most innocent girl and the man of hardened breast, felt as if the preacher had crept upon them behind his awful veil and discovered their hoarded iniquity of deed or thought. Many spread their clasped hands on their bosoms. There was nothing terrible in what Mr. Hooper said—at least, no violence; and yet with every tremor of his melancholy voice the hearers quaked.