They revealed this in an NPR interview yesterday.
Kristen Anderson-Lopez: It's funny. One of the only places you have to draw the line at Disney is with religious things, the word God.
Robert Lopez: Yeah. You just can't...
Terry Gross: You can't say the word God?
Robert Lopez: There was even a - well, you can say it in Disney but you can't put it in the movie.
Link
What about "God Help the Outcasts" from Hunchback?
^ That May have been a different circumstance given a lot of the movie took place in a Cathedral.
They cover that in the link.
The interviewer theorizes on a reason why they wouldn't let the Lopezes use the word.
I think it's a bit of a stretch, but who knows.
As they continued the interview, host Terry Gross theorized just why this might have been. Her answer wound up being the couple’s work on the high profile (and highly controversial in its own right) The Book of Mormon. This makes perfect sense on a couple levels, considering that one bad Google search might expose children (or any other sensitively minded individuals) to a translation of the most infamous song from the show, as well as trigger the outrage of Pastor Kevin Swanson and any other religious figure that connects that more off color project to the extremely tame Frozen. Further bolstering Robert and Kristen Anderson-Lopez’s case is the fact that Disney has definitely dabbled in animated films that explore themes of religion, lest we forget The Hunchback of Notre Dame.
Religion is touched upon and often visualized in other Disney movies even if they don't say "God." Mulan, Pocahontas, Hercules, etc.
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