What does the Bible say about mending a broken marriage? Where can I find this in the Bible?
My wife has an antique plate that once got broken. I was able to glue it back together and it withstands all but the closest scrutiny. A broken marriage can be like that. The crack will always be there, but can be so well glued it is unnoticeable.
You don't specify the reason for the break, and that may have some bearing on how to fix it. Also, both sides must be willing to come back together or the glue won't hold.
Unfortunately marriages do break up. Perhaps these scriptures will help the healing, as well as to prevent future breaks.
First, forgive. Luke 17:3-4 says "Take heed to yourselves: If thy brother trespass against thee, rebuke him; and if he repent, forgive him. And if he trespass against thee seven times in a day, and seven times in a day turn again to thee, saying, I repent; thou shalt forgive him." Matthew 18:21-22 says "Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times? Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven." This is unconditional forgiveness for true repentance. If the break in the marriage was because of a single adulterous act, forgive and trust that it will never happen again. And if it does, forgive again. But if the adulterous relationship was never broken off in the first place, that proves a lack of repentance. The forgiveness is not "carte blanche" for the guilty one to keep on sinning and count on forgiveness.
Second, love. Ephesians 5:25 tells husbands to love their wives. Paul concludes the section in verse 33, saying "Nevertheless let every one of you in particular so love his wife even as himself; and the wife see that she reverence her husband." In America in the 20th/21st century this almost sounds like a given. "If I didn't love her I wouldn't have married her." But this scripture was written when husbands had sometimes not even met their wives before marriage. Even if there are "irreconcilable differences" he says to love. I have called this "love even if you don't 'love'." The love Paul talks about is putting your spouse's interests and welfare first, before even your own. Paul says this is what Jesus did for the church. If both partners of a marriage are looking for ways to make the other's life the best it can be, they won't allow thoughts of adultery, jealousy, or abuse to even enter into the picture. Again, though, this is not so that one partner can say "if you loved me you would let me … ." That just shows they are not practicing the same kind of love. Both must be doing it for it to work.
Third, be honest. I'm not talking here about telling each other everything you think or feel. I'm talking about honesty in the first promise of your marriage-a commitment to stay together for life. When I hear of some people who are getting married after two or three divorces I want to grab the other person and say, "Wake up! This guy has proven himself to be a liar." I think it was Ross Perot who would not hire a divorced person for a management position because they had a proven record of dishonesty. Even for selfish reasons both parties of the contract should do their utmost to live up to it. Don't give the other person the opportunity to tell the world you were a liar. Be committed to making your vows stick.
Most importantly, give your lives and your marriage to God. It's not just a two person contract. Keep the third person (and if there are children, the fourth and subsequent persons) involved. Pray about it. It is hard to pray sincerely, and then go out and actively work to sabotage a relationship.
I am not a qualified marriage counselor. The parties in the broken marriage may want to consider talking to a Christian who is one, as many preachers are. I hope these scriptures are helpful.