I've seen religion be a factor in marriages, especially when the children are, say, four years old and older. Suddenly, the values that the parents wish to pass on, and which church they attend, go from being not very important issues to highly critical issues - sometimes with attending blow-ups and even divorces.

I'm not saying that two people who attend the same church won't get divorced. I'm not saying that at all. I'm just saying that religion can seem like a trivial issue to two married people when they get married, and as they get older and they learn more about their spouse's religion, (and more importantly) the spouses meet more of each other's congregations... well, now it might become a real source of friction.