YORK, England — Having disclosed his “sin” of masturbation, Mark Stibbe, age 17, was ordered to strip naked and lean over a wooden chair in the garden shed of a lavish Hampshire mansion on the southern coast of England.

Then came the first blow from a cane, its impact so ferocious that it sent the boy into a state of paralysis that lasted through at least 30 more strokes that left him collapsed on the floor, blood oozing down his legs.

“I remember being so appalled by how vicious the first lash was that I couldn’t scream,” Mr. Stibbe, now 56 and an acclaimed Christian author, recalled on a recent afternoon in his Yorkshire home. “You’re in this tiny shed full of canes with this man. I felt utterly powerless.”

Until that day in the late 1970s, the man he says beat him, John Smyth, was known to Mr. Stibbe and his friends as a charismatic lawyer and influential evangelical Christian leader who regularly attended the Christian forum of their nearby boarding school, Winchester College, the oldest in Britain. Now, Mr. Smyth, 75 and keeping a low profile in South Africa, stands at the center of a widening scandal of sadistic abuse of dozens of boys over three decades that has ensnared the leader of the Anglican Church, the Most Rev. Justin Welby, archbishop of Canterbury, though only peripherally.