Why did Paul quote 'the two shall become one flesh' in Ephesians 5:31?
Paul quoted 'the two shall become one flesh' to teach something higher than the human marriage union. He applied it to Christ and the church, presenting Christ as the husband and the church as the bride, so the husband-wife analogy reveals the spiritual union between Christ and His body.
Les explains that Ephesians 5:23-25 sets up an analogy: the husband is the head of the wife even as Christ is the head of the church and Savior of the body, and husbands are to love their wives as Christ loved the church and gave Himself. Then in verse 31 Paul cites the marriage text about a man leaving father and mother and being joined to his wife, the two becoming one flesh. But Paul says he is teaching something higher than even that—he speaks concerning Christ and the church. Christ is the husband, and believers are the bride. Les also connects this to the broader theme of Abraham calling God Master as servant, which drops into the human husband-wife relationship and rises again to the spiritual relationship of Christ as spiritual husband and the body of Christ as spiritual wife or bride.
For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.