The two shall become one.
From Sacred Scripture: Matthew 19:4-6
He answered, "Have you not read that he who made them from the beginning made them male and female, and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one'? So they are no longer two but one. What therefore God has joined together, let no man put asunder."
Revised Standard Version, Second Catholic Edition
From the Order of Celebrating Matrimony
The matrimonial covenant, by which a man and a woman establish a lifelong partnership between themselves, derives its force and strength from creation, but for the Christian faithful it is also raised up to a higher dignity, since it is numbered among the Sacraments of the new covenant.
The Importance and Dignity of the Sacrament of Matrimony, Introduction, Section I.1.,The Roman Ritual, Renewed by Decree of The Most Holy Second Ecumenical Council of the Vatican, Promulgated by Authority of Pope Paul VI and Revised at the Direction of Pope John Paul II