In any relationship or organization, all differences compound difficulties. One of the hardest parts of community ("unity of more than one person") is working out differences. With that foundation, every major difference that exists in a marriage compounds the potential for future difficulty - and religious difference is a major difference.
Can it be overcome? Absolutely. Is it easy? Absolutely not! (especially the stronger the beliefs and difference are) It is the #1 breaker of marriage I know for those whose religious beliefs differ in significant ways.
Having said that, the following are multiple sides of the issue:
1) Based on the most recent LDS divorce stats of which I am aware (and, to be totally honest, I have not researched this in over a year), the temple marriage divorce rate is about 6%-10% - the lowest rate of all religion-based rates in the US, by far. The divorce rate for LDS non-temple marriages (when both people are Mormon) is about 18%-21% - dead average for all Christian denominations. The divorce rate for an LDS/non-LDS marriage is about 41%. That's a HUGE difference, and it's the highest rate among all of the religion-based categories.
Basically, a Mormon who marries "outside the faith" is 4-7 times more likely to get divorced than someone who marries in the temple and over twice as likely as someone who marries another member outside the temple. That's significant, to put it mildly. Finally, a 41% divorce rate means a 59% non-divorce rate - but there is no indication of how many of those "successful" marriages only stay intact because the Mormon spouse gives up attending for the sake of the marriage. If you add those two aspects (divorce from the marriage AND divorce from the Church combined), my guess is that the actual "failure rate" (from a strictly religious activity standpoint) probably is as high as 70%. That's frightening.
2) My own "courtship" was not perfectly in line with the Church's general standard - on the other extreme. I never dated anyone other than my wife after she turned 16. I proposed to her (and she wore an actual engagement ring, with the wedding band in her room at home) before my mission - and before her senior year in high school. She was 17 at the time. We got married 6 weeks after I returned from my mission, a few days after she turned 20. We had all kinds of pressure from disbelieving friends and family to slow down and not make that commitment at that age. We understood the concerns as well as we could, and we understand them much better now that we have kids who are past those ages, but we were convinced we were doing what we were supposed to do - and we were right.
3) Inter-faith marriage, when both spouses have strong beliefs that differ, can be and usually is VERY hard, and it is something I believe should NEVER be done with the idea that "s/he will convert after our marriage". That belief leads to actions that lead to pressure that leads to hard feelings that lead to divorce, in my opinion. If you really love someone, you love them not only for who they might become but also for who they are - regardless of who they might become.
4) Having said all that, in the end, I also believe that if two people truly can become one in this life, God will not split them up in the hereafter - since "joining the Church" is not the ultimate goal, and since we symbolically seal them regardless of their denominational affiliation in mortality. Becoming one with each other and with God is the ultimate goal - and that happens outside the LDS Church and outside Christianity all the time. It's just much harder when religious beliefs differ.
Summary: I have no answers for individual circumstances, but, as a general rule, I believe strongly that it is MUCH easier to make a marriage work when foundational religious beliefs are shared. If they aren't, based on real statistics, it becomes a total crap shoot. Some win; at least as many lose.
“I Take Up My Pen”: Florida Mission, 1962
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