One of the hardest challenges to make a marriage work is fundamental differences in opinions. A Catholic and a Jew marry, and only serious love and serious respect will prevent the Jew from trying to convert the Catholic and vice versa. Same with a Muslim and a Hindu, or any religion and an Atheist. Even a tolerant member of any of these religions will rapidly get fed up if the other partner in the relationship is not so tolerant and starts forcing his or her beliefs on the first partner. 

My parents are a good example of when this works. Mom and Dad are about as opposite politically as you can get. Mom is a Flower Child Liberal. Dad is an Ex-Marine Conservative. Couldn't be more different, but they've stuck together for 4 decades (and had me!). How? Well, let's just say the closest they ever came to splitting up was when Dad got all foul about Clinton's win in '92. Mom damn near left him over the asshattery. 

He cut that out real quick when she threw the gauntlet down.