George Carlin was raised a Catholic, and his Catholicism was a large part of his history and his being. However, in adulthood, especially as he got older, he recognized that organized religion and people who used their faith to condemn and control others was wrong. Listening to this interview, I think he was a pragmatist. He just didn't feel that the whole God question was important to his life.
He might not have been an atheist, but he certainly wasn't a supporter of religion. He would fall firmly into that 1/4 of Americans who say they belong to no religion.