Parker is an American actor, voice artist, animator, screenwriter, director, producer, and musician, best known for being the co-creator of 'South Park'; his work on the films 'Orgazmo', 'Team America', and 'BASEketball'; and the Broadway musical, 'The Book of Mormon'. 
During a 3/11/11 interview with Vulture, "Trey Parker and Matt Stone Talk About Why 'The Book of Mormon' Isn't Actually Offensive and the Future of 'South Park'", Matt Stone states Trey Parker is an atheist and Trey Parker (also present) does not object:
Stone: "We’re essentially atheists — I mean, I am; Trey I don't want to speak for. But coming from that point of view, we’re atheists who don’t hate religion, who are kind of fascinated by it and kind of admire it. [We thought], What would that look like? What would an atheist love letter to religion look like? And I think that’s what this musical is. Now, I don’t think that every Mormon will necessarily like what this love letter says, but it’s our version of, “Hey, we think religion is really cool, here’s what we think about it.” And it’s a musical, so it’s gotta have a feel-good end, and it’s gotta have a big heart, a big story. And that’s the only way to really tackle talking about religion in narratives, is treat the people in them like really good people who are trying to do the right thing. Instead of like, “Look what they do at the tip of a gun, fucking religion.” That’s like, you’re obviously not interested in having a real discussion about it. And that wouldn’t be a very fun musical to watch." 
In the commentary for the South Park episode Go God Go (from November, 2006), which features Richard Dawkins, Stone talks about how they have been wrongly perceived as atheists. He paraphrases an interaction he remembers in their Nightline interview: Interviewer: So, you guys are atheists?...because you've made fun of religion a lot on the show. MS: What? No. We're not atheists."