When the Catholic Church teaches that marriage is a Christian vocation it is saying that the couples relationship is more than simply their choice to enter a union which is a social and legal institution. In addition to these things, marriage involves a call from God and a response from two people who promise to build, with the help of divine grace, a lifelong, intimate and sacramental partnership of love and life.
The Second Vatican Council teaches that all Christians in whatever state or walk of life are called to the fullness of Christian life and to the perfection of charity (Constitution on the Church, n. 40). The call to marriage is a particular way of living the universal call to holiness given to every Christian in the Sacrament of Baptism. The calls to priesthood, or to the vowed religious life, or to the single life are other Christian vocations. Along with marriage, all of them equally though in different ways, are a response to the Lord who says, Follow me.
The call to love is the fundamental and innate vocation of every human being. In the vocation of marriage something which is written in the very nature of man and woman, we see that the love of husband and wife becomes an image of the absolute and unfailing love with which God loves (#1603 and 1604).
A vocation is a personal call. It is offered freely and must be accepted freely. Attraction to a certain way of life or to a specific person can be a good sign of being called. Most often a person comes to recognize and accept a vocation gradually. This process, sometimes called discernment, is an opportunity for growth. It can be helped by prayer and guidance from trusted mentors, friends and family.
However, what begins as attraction must deepen into conviction and commitment. Those who are called to the married life should be ready to learn what their vocation means and to acquire the virtues and skills needed for a happy and holy marriage.
The vocation to marriage is a call to a life of holiness and service within the couples own relationship and in their family. As a particular way of following the Lord, this vocation also challenges a couple to live their marriage in a way that expresses Gods truth and love in the world.