Source, in full
A misquote. It derives from an interview that journalist Bruce Cook conducted with Kerouac in 1968 and reported in his book The Beat Generation (1971). According to Cook, Kerouac explained to him his method of writing: "I'll just sit down and let it flow out of me ... It's the Holy Ghost that comes through you. You don't have to be a Catholic to know what I mean, and you don't have to be a Catholic for the Holy Ghost to speak through you." Source of misquote.