Yes, he has claimed many times that he is an atheist.
Brian the Bachelor
Brian Griffin
A Woody
Brian
Brian is in every episode
He's a dog, a failed writer, a drinker, a good singer, an atheist & he's terrible at relationships.
Brian the Bachelor
Brian Griffin
No.
Brian the Bachelor
A Woody
Brian Does Hollywood
No. Seth MacFarlane is an atheist.
Brian: Portrait of a Dog
Brian has a liberal political philosophy
Brian the Bachelor (Season 4)
Brian & Stewie's Emmy appearance & performance wasn't part of any Family Guy episode.