I liked this term so much when I heard it preached by Tim Keller in his sermon on marriage. I have not heard such metaphoric illustration on marriage of a Christian married a non Christian or someone by name Christian. But it won’t apply if you yourself is a nominal Christian, as that would require a separate conversation.

What does that mean exactly? 

Not Good for Man to be Alone 
This is from God. God created Adam and let him rule over all the creations and name them all. 

And out of the ground the LORD God formed every animal of the field and every bird of the sky, and brought them to the man to see what he would call them; and whatever the man called a living creature, that was its name. The man gave names to all the livestock, and to the birds of the sky, and to every animal of the field, but for Adam there was not found a helper suitable for him. So the LORD God caused a deep sleep to fall upon the man, and he slept; then He took one of his ribs

The man Adam had all the power to name animals etc but there was not a helper suitable for him. Suitable. Do you think this is what we call compatible relationship? When a Christian looks for a girlfriend/ boyfriend, does that suitable word come to mind? What you think as suitable must be scrutinized by the holy scriptures of God. Adam looks around and found no one suitable . He feels lonely. And God knows it. He said it’s not good for the man to be alone. He needs a helper. So God did this amazing miraculous work by putting Adam to sleep then miraculously pulled out a rib bone to create a woman. It’s to be noted that God created a man initially this way: Genesis 1:26

Then God said, “Let Us make mankind in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the livestock and over all the earth, and over every crawling thing that crawls on the earth.” So God created man in His own image, in the image of God He created him; male and female He created them.

It says here God created man and woman. That’s a big brush but the details comes in chapter two when God pulled out a rib from Adam and created Eve. So when God created Adam, 

Genesis 2 V18 So the LORD God caused a deep sleep to fall upon the man, and he slept; then He took one of his ribs and closed up the flesh at that place. And the LORD God fashioned into a woman the rib which He had taken from the man, and brought her to the man. Then the man said, “At last this is bone of my bones,
And flesh of my flesh; She shall be called ‘woman,’ Because she was taken out of man.”

Woman was made out of a man. Adam said Eve is flesh of his flesh, and then God said the man shall leave his parents and become one flesh. That’s exactly what they were to begin with . Man and woman were one flesh. One flesh is the will of God the will of Gods creation. The beautiful creation of God is one flesh . 

We must get back to one flesh on a marriage. When God brought Eve to Adam, that is the first marriage in the universe. 

One Flesh

Genesis 2: 23-24Then the man said, “At last this is bone of my bones,
And flesh of my flesh;She shall be called ‘woman,’Because she was taken out of man.”
For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh.

This one flesh must be initiated by God. And it’s between the opposite sex. God’s original plan is a man and a woman. Different genders.

Finally you can’t be one flesh if you have a spouse who doesn’t believe in Christ or Christian by name. Then you would end up putting Jesus in the suburb when you live in the city. One flesh means you share everything in life permanently and legally. Thats really the vow a couple takes in marriage, the old vow. But its powerful. Thats really the meaning of One Flesh, sharing all of your life, being vulnerable and open and helping to sanctify one another.

Ephesians 26 ²⁵ Husbands, love your wives, just as Christ also loved the church and gave Himself up for her, ²⁶ so that He might sanctify her, having cleansed her by the washing of water with the word, ²⁷ that He might present to Himself the church ^[q]in all her glory, having no spot or wrinkle or any such thing; but that she would be holy and blameless. ²⁸ So husbands also ought to love their own wives as their own bodies. 

As Christ sanctifies the church as the bridegroom, husband is to sanctify the wife and cleanse her with the word of God, so that she would be holy and blameless. This is a high call for the husband. But who can speak right into husband’s life or wife’s life apart from his/her spouse? That’s why sanctification can happen in the right marriage dynamics like nothing else.
All these can only happen, if we have Jesus’ right in our marriage, i.e. Jesus must be with us and not put in the suburb when live in the city, or the other way round.