On New Year’s Day in 1961, Leary’s team ingested mushrooms in his living room. “Such a sense of awe,” Professor Smith said afterward. “It was exactly what I was looking for.”

A year later, the group gathered in a church basement as a Good Friday service was being held upstairs and tried an experiment involving 20 volunteers in which half were given the psilocybin mushrooms and the other half a placebo. Professor Smith received the drug, which was legal at the time, and reported that he was certain he had had a personal experience with God. He thought that the voice of a soprano singing upstairs was surely that of an angel.

“From that moment on, he knew that life is a miracle, every moment of it,” Don Lattin wrote in “The Harvard Psychedelic Club,” a 2010 account of the psychedelic research project, “and that the only appropriate way to respond and be mindful of the gift of God’s love was to share it with the rest of the world.”

Professor Smith later became disenchanted with Leary’s “tune in, turn on, drop out” gospel, but he retained his belief that the briefest of insights from a psychedelic trip could be mind-expanding.

Those early drug experiments, however, were enough for him, he wrote in “Cleansing the Doors of Perception: The Religious Significance of Entheogenic Plants and Chemicals” (2000). (The word entheogenic refers to substances that produce an altered state of consciousness for spiritual purposes — “God-enabling,” in Professor Smith’s words.)

“If someone were to offer me today a substance that (with no risk of producing a bummer) was guaranteed to carry me into the Clear Light of the Void and within 15 minutes would return me to normal,” Professor Smith wrote, “I would decline.”

Huston Cummings Smith was born to Methodist missionaries on May 31, 1919, in Suzhou, China. The family soon moved to the ancient walled city Zang Zok, a “caldron of different faiths,” he wrote in his 2009 memoir, “Tales of Wonder: Adventures Chasing the Divine.”