William Slaone Coffin was an author and theologian and minister and social activist who died last year. He was a military intelligence officer in WWII after which he became a minister. He was a Freedom Rider during the Civil Rights era, chaplain at Yale during the Vietnam-era and organized protests against the war, and later became the minister at Riverside Church in New York. He even inspired the character of Rev. Sloan in the comic strip Doonesbury.
In my search for the source of Coffin's quotation--I never did find when/where he actually used it first, although it is the title of a documentary about Coffin, maybe I need to see it--I came across two interviews of Coffin that are worth reading, watching or listening to.
1. An interview with Bill Moyers on the PBS show NOW where Coffin talks about his impending death.
2. An interview on Religion and Ethics News Weekly
Grace and Peace,