A professional baritone, Laurence Gien has sung in some of the most storied opera houses in the world. So, he wasn’t particularly daunted by the glitter and gold of St. Peter’s Basilica.

But the message he came to deliver to an audience that included Pope Francis and dozens of bishops and cardinals — of his sexual abuse as a child at the hands of a priest — made his appearance a powerful experience nonetheless.

Speaking as part of a solemn ceremony on Tuesday evening, during which Francis begged forgiveness for a host of sins, “was amazing for my own spiritual journey as a human being,” Mr. Gien said in an interview in his Rome hotel the next morning.

It was the first time a survivor of clerical sexual abuse had spoken in a formal prayer service in the basilica, a Vatican spokesman said.

His operatic voice booming in the cavernous basilica, Mr. Gien, 63, recalled how as an 11-year-old child he had been groomed over several months by a priest at his boarding school in South Africa until, one morning, “in the screaming silence, he took from me what should never be taken from any child.”

“This moment in time, in all its sordid detail, is a part of my physical being and consciousness, and is as present today as it was when it took place,” he said.