Vonnegut was named 'Humanist of the Year' in 1992.
Vonnegut is quoted from an interview in Free Inquiry magazine: "For at least four generations my family has been proudly skeptical of organized religion."
In Palm Sunday (p 210, paperback) in a speech he says "You have just heard an atheist thank God not once, but twice. And listen to this: God bless the class of 1974." On page 327 he calls himself a "Christ-worshipping agnostic." He also makes several Biblical references in the opening pages of Player Piano.
A quote from his autobiography Palm Sunday, reprinted from a "self-interview" printed in the Paris Review:
INTERVIEWER asks something about how he was affected by his study of anthropology.
VONNEGUT: Yes, it confirmed my atheism, which was the religion of my fathers anyway. (He was from a family of pioneering American freethinkers.)
I am an atheist (or at best a Unitarian who winds up in church quite a lot). -- Kurt Vonnegut, Jr. Fates Worse than Death: An Autobiographical Collage of the 1980s