At his son Christian's murder trial a few years ago, Brando refused to take a religious oath, stating that he is an atheist.
A reader disputes the cite above, providing this account:
I believe this is in error. During a news broadcast several years ago, I remember seeing him being sworn in at a trial. I don't know if it was his son's trial or another, but in the clip, he refused to swear on the bible, saying something along the lines of:
"while I do believe in god, I do not believe in the same way as others, so I would prefer not to swear on the bible".
In his autobiography, Brando describes seeing his parents together in the afterlife in a dream after his father died. He also claims to share beliefs with Native Americans on spirituality.