The 'Ocean's Thirteen' star was brought up a Southern Baptist by his parents in Missouri, but abandoned his faith when he started college and "discovered himself".
Brad told Parade magazine: "I didn't understand this idea of a God who says, 'You have to acknowledge me. You have to say that I'm the best, and then I'll give you eternal happiness. If you won't, then you don't get it!'
"It seemed to be about ego. I can't see God operating from ego, so it made no sense to me.
"When I got untethered from the comfort of religion, it wasn't a loss of faith for me, it was a discovery of self."
The 43-year-old actor also revealed he finds it insulting that people think it was girlfriend Angelina Jolie who introduced him to charity work.
Brad - who founded his own organization to rebuild homes for victims of the New Orleans disaster - said: "It's idiotic to say my relationship got me into charity work!
"I do it because I'm a member of the human race. In Africa you see people on the street dying from AIDS, children left without parents. We're all cells of one body, with the same emotions and desires for our families - for a little dignity and a chance for a better life. Let's focus on that!
"I believe in the founding principles of America. I want to fight for that. I know most Americans feel the same way.
"Whoever said all men are born equal never left his own backyard. I see people everywhere without opportunity. I want to help level the playing field."
The 'Se7en' star is now father to three adopted children, Maddox, six, Pax, three and Zahara, two as well as 16-month-old daughter Shiloh with Angelina.