Soon to be divorced guy weighing in on the marriage topic, but not the religion topic.
Marrying young, as someone else mentioned, is a big part of it. We were married at 21 and made it to 37, in retrospect partly due to sheer persistence.
Respect and trust are key, because those two give you the motivation and the opportunity to really communicate. Almost after the fact I have learned that real talking sometimes breaks down due to a lack of either or both of those two ingredients and you lose someone without even realizing it.
There are stresses put on people by life, and not working through it can drag even the best of us down. We had a few doozies, more than some but fewer than others, I'm sure.
Unsolicited advice:
1. Don't take your spouse for granted. I thought our marriage was good when in reality I wasn't paying attention to signs of trouble (because I was focused on other stuff -- work, triathlons, money, etc.).
2. If you're wondering about the stability of your marriage, don't have kids. Divorce sucks, and it especially sucks for kids, who I consider innocent civilian casualties.
2Cor521