Dr. Grant Wacker is a historian specializing in the history of Pentecostalism, evangelicalism, world missions, and American Protestant thought. He is a graduate of Stanford (BA, Philosophy) and Harvard (Ph.D., Religion), and serves as the Gilbert T. Rowe Professor Emeritus of Christian History at Duke Divinity School in Durham, North Carolina.

He is the author of several books, including Heaven Below: Early Pentecostals and American Culture, Religion in Nineteenth Century America, and Augustus H. Strong and the Dilemma of Historical Consciousness. His writing has appeared in The Christian Century and Christianity Today, and he has served as editor on numerous books concerned with the intersection of American culture, religion, and history.

His newest book is a biography of Billy Graham titled One Soul at a Time: The Story of Billy Graham, though it is not his first book about Billy Graham. In 2014, he published America's Pastor: Billy Graham and the Shaping of a Nation, an interpretive study concerned with the overlap of Graham's own person and the forces that shaped modern America during the last century.

In One Soul at a Time, Dr. Wacker turns his unflinching-yet not unkind-interpretive focus to Billy Graham the man himself, as he lived and moved and breathed, as he evolved decade by decade. It is an honest portrait of a man historian Steven Miller once called "America's most complicated innocent," a man who likely preached to more people than anyone else in history.

Dr. Grant Wacker and his wife, Katherine, live in Chapel Hill, North Carolina, where they are lay members of Orange United Methodist Church.