The actor said he has been deeply affected by the film that tells the story of a 2001 news investigation that exposed a child sex abuse scandal in the Church in Boston in the United States.
The 64-year-old plays Boston Globe journalist Walter "Robby" Robinson, who led a team of reporters to expose the abuse.
Speaking about a scene in which Rachel McAdams's character tells her devout grandmother about the reality of her church, he said: "My mother was a devout catholic and I was an altar boy.
"My mum went to mass every day and when I think of what (those revelations) would have done to her... I think that's big, it's one thing to do, but crushing someone's faith is another level."
The actor admits he still "drops in" to church and said the film was not aimed at picking on the Catholic faith.
He said: "I haven't been Catholic since I was an altar boy, there's no pretending, I'm lapsed... But as much as I hate what's happened in the world based on organised religion and people's alleged faith, I'm a defender and think it's good for people.
"I'm totally cool with my vision of what people call God and I'm good there, but what this movie's about is not religion, but institutions. It's about people who take power and seek power and use that power to disadvantage the disenfranchised and the powerless and it happens in a lot of places, literally all over the world."
Mr Keaton said he was excited about how powerful journalism could be: "That's what we hope younger people, when they see this movie, can get excited about and feel the power."
The film has been nominated for six Academy Awards, including best picture, best director and best supporting actress for Rachel McAdams.
Spotlight is released in UK cinemas on January 29.