I have received the two following related questions and thought I would answer both together.
Where in the Bible does it talk about what constitutes a marriage in the eyes of God for today? Is co-habitation permissible in the eyes of God if people think they are Christians?
LA from Centennial, Colorado
I am married but not by a church. Is that a sin?
C. from Chicago, Illinois
What constitutes marriage? (Part 1)
God created marriage in the Garden of Eden - that's how old it is. The way he designed it was so perfect that it holds up even, as you asked, "for today".
Adam intuitively understood marriage and his wife Eve from the beginning. As God brought her to him Adam said; "'This is now bone of my bones, and flesh of my flesh." (Genesis 2:23) Then scripture adds this admonition: "For this cause a man shall leave his father and his mother, and shall cleave to his wife; and they shall become one flesh." (Verse 24)
At its core, marriage is a covenant between a man and a woman to live faithfully with each other for life in intimacy so deep they are no longer two but one person. Marriage sets "family" in motion which is the bedrock of society.
Jesus confirms the holiness and call of marriage in the New Testament by repeating the scripture from Genesis in Matthew 19:5. He decries the popular divorce of his day by telling them that was not God's original plan and adding; "'What therefore God has joined together, let no man separate.'" (Matthew 19:6) That tells us that God participates in marriage; he listens carefully as vows are made and he blesses unions which seek to place him at the center of the relationship.
Co-habitation is not marriage. Sexual relationships outside marriage are considered sin. Sexual relations with a married person is called adultery but if the sex is between unmarried partners the sin is called fornication.
There are many passages in the Bible which speak about both adultery and fornication but here is a typical one that makes things pretty clear. "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints." (Ephesians 5:3)
In answer to your second question then, co-habitation is not permissible and that would be true whether people consider themselves Christians or not. God instituted marriage before he gave the law through Moses and certainly before Christianity existed. People may ignore God's plan because he has given us free will but he meant his plan for everyone.
Marriage was God's gift to us to provide love and safety as we live out our lives. (Please see "Happy Marriage-Best Rule" for additional information on living together.)
Next: Must you be married in church?
Does it matter "how" you are married? (Part 2)
The details of "how" people marry vary in different cultures and times in history. Some are elaborate rituals and some are simple and that is okay because God doesn't require any standardized procedures.
The two people marrying are actually the ones performing the wedding ceremony. They are making sacred vows to each other which constitutes the core of the union. The "officiator" such as the priest, minister, Judge or Justice of the Peace is mainly the witness to the vows (as are any guests). They represent the public interest in the marriage and attest to the legality of it for society's sake.
Therefore, is it a sin if you are not married in church? I don't think so. Did you marry genuinely from your heart? Is your spouse flesh of your flesh and bone of your bone? Was your intention to be one flesh and live faithfully together until death parts you? If so, I see no sin. Did you abide by the legal rules within your state of residence? Normally, if the state sees you as married, I believe God sees you as married.
However, if you feel you missed anything by the way you were married, fix it. If your marriage was purely a secular event you may want to now renew it in church to acknowledge or affirm God at the center of it.
Churches have much to offer as the setting for marriage and ideally that is where I think they should take place. A church treats a marriage spiritually and invites God's presence and blessing. If you feel a tug in your heart, go to your church and explain your situation and arrange to have a second ceremony.
If you broke the rules of your particular church denomination about the proper way to marry you will have to decide how important that is to you. Maybe you will decide to jump through any hoops they require to "validate" your marriage and that's fine. Realize though, it has nothing to do with the truth of the matter. In your heart, you know if you are married and God knows. His view is the only thing that really counts.