The mystery of marriage as Paul explains, is one we should learn from the union between Jesus and the church. 
As Paul reveals, the institution of marriage did not begin on earth but is a sort of future reality God brought into man’s experience (Ephesians 5:22-33). When God made man and woman, He had something in mind. He planned that the two (though separate entities), through the mystery of marriage would attain spiritual unity.
Nevertheless, there’s a part of the statement we may miss when we read this text through the lens of familiarity. Yes, God says the two will become one, but only if they hold fast to each other. This is the secret to becoming one, which means a unification of spirits. This translates to oneness in everything that counts; oneness of the flesh. Without the realization of the unified man and woman, it’s not really a marriage.
Marriage is a big decision. A man must come to the great decision of leaving his father and mother, and so must the woman. This may also figuratively imply an independent stance, where the man and woman cling to each other and find completion in their union. Of course, they don’t become estranged from family and friends but now have a new order of priority putting each other above everything else but only second to God.

Dear Heavenly Father, thank You for the institution of marriage. Thank You for Your wonderful plan that unites my partner and I. Lord, we ask for the grace to hold fast to each other every day, no matter the challenges that come our way. Grant us the grace to make our marriage the perfect picture of what You intended. Please give us the wisdom to navigate the currents that fiercely try to devastate our love, joy, and peace. We offer everything to Your wise leadership, because You alone can make it perfect.  
In Jesus’s name I pray. Amen.