Yes, it is a "mystery" how a husband and wife are two separate people but joined as one in marriage. This certainly shows God's viewpoint of the sacredness of the marriage covenant and, more importantly, the special nature of the marriage union. A verbal pledge and vow is no more worth than a marriage contract signed by both parties. Words and paper do not make a marriage. Their is a "pledge of the heart" that gets to the core of a sacred union. It's the same with our salvation and union with Christ. Salvation is far more than just a prayer and a public confession. We already know that even baptism and practicing the Lord's Supper do not make salvation. Their is a union of heart to heart, spirit to spirit between the sinner and his/her Savior. Salvation is one because the hearts become as one in Christ. A marriage is the same. The heart of the husband united with the heart of his wife bring about the oneness they have in marriage.
Christ and His Bride, the Church, are seen as ONE in His sight. And that means one with the Father, the Son, and the Holy Spirit. Note John 17 and Ephesians 1:13-14. There is absolutely no union tighter than that.