Religion should be number 1. Everything else probably follows that path.
The most stable, happy marriages that you will see have two people who share a belief system and respect one another not just in word but in deed. That should be blindingly obvious, but here we are.
Figuring out if a potential life mate shares your belief system and will respect you is probably most easily done by observing the way they practice their faith, but some people are really good at not revealing or not understanding their true selves until after marriage. Still, if you start a relationship with someone who already can demonstrate respect and share a belief system, you are far more likely to succeed than if not.