Develop an understanding of each other's religions, communicate personal beliefs, and be able to embrace each others' differences. Not in a way to spark debate or controversy, but to understand and appreciate their own beliefs. Also, communicate as to whether these differences would be hindrances to the long-term of the relationship.

Not an exact example, but my ex-fiance is a devout Catholic and her family is about as traditionally Catholic as it gets, and I'm Methodist with a much more mild belief system and comprehension of Christianity.

At first, it was something we didn't communicate our differences on until they came up in conversation or situationally and would create some small rifts between us.

When we got engaged and started planning wedding stuff, the conversation started as to "when" I would convert to Catholicism and that was not something we ever discussed and not really something I was willing to do on a whim (I was going to, but relationship ended for other reasons).