Marriage is ordained by God, He created it, therefore it would make perfect sense that he has created the manual for it as well.
Now marriage is not easy or perfect and it will never be perfect in this world, since it involves the union of two sinful people trying to survive in a sinful world.
But it does have a purpose and that is to grow as individuals and as a couple and to draw closer to God through our experiences, our difficulties and tribulations.
If you understand and know God then you know our purpose here in this world is to glorify Him through our good works and that He uses circumstances and the people around us to mold us into his image, and this includes your spouse.
Now God is true to His Word, so even if you don't believe in God, if you obey His Words then you will receive the blessing from your obedience. For example, the Bible says, Husbands love your wives and Wives respect your husbands. If a couple that doesn't believe in God follows this rule, then God will be true to His Word and they will be blessed. Which is the reason why even a couple that doesn't believe in God can have a happy marriage. Or a couple that believes in God can end up in divorce.
It's not believing in God that makes the difference in your marriage, it's obeying what God said.
Another example, God says do not commit adultery and that even looking at another woman and desiring to have her in your heart, is already adultery. If a couple doesn't believe in God but decides to follow this moral principle. They will receive the blessing from it, because God is true to His Word, and if a couple that believes in God doesn't follow this moral principle then they will face the consequences. It's that simple, it's not just believing it's obeying and applying.
So this is the reason why many couples that believe in God end up in divorce. It's not God who fails them but the real reason is that they didn't follow what God ordained for marriage. It's also the reason why many couples who don't believe in God seem to be happy, because even though they do not believe they follow the simple moral virtues that God ordained.
What are God's simple rules for marriage:
1) Love each other. If you love each other you wouldn't want to hurt each other.
2) Husbands love your wives. A woman wants to feel loved. She wants to feel as if she is the most important person in your life. She wants to be wooed, romanticized and swept of her feet, even after many years of marriage.
3) Wives respect your husbands. A woman wants emotional love but the way a man understands love is through respect. Make him feel like a man, like a king, don't belittle him and make him feel like you have no respect for him or for what he does.
4) Let no corrupt words come out of your mouths but that which is necessary to edify the listener. Speak words of encouragement to each other, speak that would build not words that hurt or destroy.
5) Submit to one another, get rid of the mentality that I'm better than you and you should submit to me. Learn to give and take. Learn to come to agreements when you can't agree on a subject.
6) Your body no longer belongs to you but to your spouse, so don't deny sex to your partner, unless your fasting or you have a serious medical condition, in which case you should seek medical assistance. But remember this doesn't mean that since your spouse's body belongs to you, you can do with it whatever you want, always keep in mind the first rule which is to love one another and not to cause each other harm.
7) Forgive one another. Remember you can also make mistakes, so forgive the way you expect to be forgiven. Don't hold grudges and try not to go to bed angry, if possible resolve your disagreements the same day.
8) Be kind and gentle with each other. Correct each other in gentleness, realizing that you can also make mistakes.
9) Enjoy life together, don't enjoy life by yourself or with your friends, enjoy it with your spouse. Also don't take life to seriously, enjoy it, but enjoy it with your spouse. Have fun together.
This are some simple rules for marriage that are all found in the Bible. God ordained that if obeyed and applied to your daily marriage life, you will achieve happiness together.