Kelly was born in 1893 in Chillicothe, Ohio, to a Quaker family (members of the Religious Society of Friends).
The branch of Quakerism in which he was raised (Wilmington Yearly Meeting) had been influenced by the 19th century revivalists and worship services were similar to other low-church Protestant groups.
When World War I broke out, he signed up to work for the YMCA with the troops in training at Salisbury Plain in England.
He and his wife then went to Berlin and worked with the American Friends' Service Committee in the child feeding program, where they were instrumental in founding the Quaker community in Germany.
[4] He published the dissertation for his second doctorate in 1937, but he failed in the oral defense due to a memory lapse;[3] this failure put Kelly into a period of grief, during which time he apparently had a spiritual awakening.
[1] Kelly received word on January 17, 1941, that Harper and Brothers was willing to meet with him to discuss the publication of a devotional book.