From Ephesians by Peter S. Williamson, reflecting on Ephesians 5:32:
Marriage, a sacrament. Although the New Testament teaches about marriage in a variety of places, Eph 5:32 is the primary basis of the Church’s recognition of Christian marriage as a sacrament. Sacraments are “efficacious signs of grace, instituted by Christ and entrusted to the Church, by which divine life is dispensed to us” (Catechism 1131). How then is marriage “efficacious” and how does it dispense divine life?
Before the coming of Christ, God’s purpose for marriage was often thwarted due to the hardness in human hearts (Mark 10:5), and for this reason the law of Moses permitted divorce (Deut 24:1). The good news is that in the New Covenant through the gift of the Spirit, Jesus removes our “stony hearts” (Ezek 36:25–27) and makes us capable of fulfilling God’s will (Rom 8:4), including lifelong marriage. “By coming to restore the original order of creation disturbed by sin, [Jesus] himself gives the strength and grace to live marriage in the new dimension of the Reign of God” (Catechism 1615).
How is this accomplished? When a woman and a man give their consent before the Church, when the Holy Spirit is invoked on the couple through prayers and blessings, “the spouses receive the Holy Spirit as the communion of love of Christ and the Church. The Holy Spirit is the seal of their covenant, the ever-available source of their love and the strength to renew their fidelity” (Catechism 1624, citing Eph 5:32). In other words, the husband and wife become participants in the love between Christ and the Church—they become capable in a new way of drawing on that powerful divine love that surpasses human strength.