EXCLUSIVE

Katie Holmes filed for divorce primarily over Tom Cruise's fierce ties to Scientology, fearing that Tom would drag Suri deep into the church ... sources connected with the couple tell TMZ.

Our sources say ... Katie has never been fully committed to Scientology, even though she has had ties with the church. We're told the couple had been arguing over Suri -- that she's now of the age where Scientology becomes a significant part of her life.

Sources familiar with the split tell us Katie is filing for sole legal custody -- as we first reported -- specifically because she does not want Tom to control decisions relating to religion.

Sources have told us that the divorce is NOT about money -- we're told Katie doesn't need Tom's money ... because she has plenty of it ... and will be well taken care of under the terms of the prenup.