Someone was asking recently whether it is true that there are only two vocations–marriage and religious life–and that being single isn't really a vocation.
Actually, when you get down to it, there is only one vocation: the vocation to love. In Familiaris Consortio, Pope John Paul says:
11. God created man in His own image and likeness(20): calling him to existence through love, He called him at the same time for love.God is love(21) and in Himself He lives a mystery of personal loving communion. Creating the human race in His own image and continually keeping it in being, God inscribed in the humanity of man and woman the vocation, and thus the capacity and responsibility, of love and communion.(22) Love is therefore the fundamental and innate vocation of every human being.
The last sentence is quoted in the Catechism of the Catholic Church, n. 2392.
Now, what are the basic ways we fulfill that vocation? Love has to be self-giving, and if it is to be complete, it has to involve the whole person, body and soul. So our sexuality has to be included in the way we live out our vocation to love. Moreover, self-giving is most complete when we give not only the present moment, but also our future lives, so far as we can–the love should include commitment.
For these reasons, the normal ways of fulfilling the vocation to love are (1) marriage, and (2) dedicated virginity or celibacy–a committed single state.
Pope John Paul II continues in Familiaris Consortio:
Christian revelation recognizes two specific ways of realizing the vocation of the human person in its entirety, to love: marriage and virginity or celibacy. Either one is, in its own proper form, an actuation of the most profound truth of man, of his being "created in the image of God."
I say these are the normal ways, because there might be circumstances that would exclude a full dedication to either way of life: if men or women are unable to marry, yet their spiritual director and confessor urge them not to promise virginity or celibacy. I would think it rare that such advice would be right for a person's whole life, but can't be excluded. God permits some to have bodily sicknesses so severe that they can't live a normal life, and similarly God permits some to have psychological sicknesses of a nature that they can't fulfill their vocation to love in the normal way–it is to be fulfilled in acceptance of their sickness, and in doing what they can to live according to love and grow in it.
So, it is right that as a rule you can't choose as a vocation an uncommitted single life (though in some cases one might have to accept it, at least for a long time). But you can choose a committed single life, by vows or some other promises dedicating one's single state to the service of God, one's neighbors and the Church. Such a life committed to celibacy or virginity for the sake of the kingdom is possible, however, outside a religious community, and therefore it's not entirely correct to present the two alternatives as "marriage" and "religious life." Pope Pius XII writes in his encyclical on consecrated virginity:
While this perfect chastity is the subject of one of the three vows which constitute the religious state, and is also required by the Latin Church of clerics in major orders and demanded from members of Secular Institutes (Cf. apostolic constitution Provida Mater, art. III, section 2), it also flourishes among many who are lay people in the full sense: men and women who are not constituted in a public state of perfection and yet by private promise or vow completely abstain from marriage and sexual pleasures, in order to serve their neighbor more freely and to be united with God more easily and more closely. (Sacra Virginitas, n. 6)
There is of course a connection among all the counsels, and it is quite natural (even to some extent necessary) that a person who commits himself to virginity or celibacy for the sake of love will give himself over to the spirit of the other evangelical counsels, poverty and obedience. Quite possibly he will also embrace the actual practice of these counsels. But this actual practice of the counsels could happen at a later stage, and does not hinder the state devoted "to the Lord" (1 Cor 7:35) in committed virginity or celibacy from being a vocation.
Sometimes the vocation to a single life dedicated to service is discovered only through the fact that marriage is not impossible. In my book Paths of Love: The Discernment of Vocation, I explain it this way:
A second way in which a way of life may be found by divine providence is when providence determines the way of life. Thus, if one believed that one was called to religious life, but due to an incurable case of severe depression, one was unable to enter religious life, then one should accept this as God’s will, and turn one’s attention to other ways of life. Or again, someone might believe that he should marry, but find himself unable to do so. In such a case, he could then accept the situation as appointed by divine providence, and embrace the single state as a means of serving God and neighbor. Thus Pope Pius XII, speaking of the various ways in which a vocation to virginity may be experienced, includes the example of a woman who wants to marry, but is unable to do so.
When one thinks upon the maidens and the women who voluntarily renounce marriage in order to consecrate themselves to a higher life of contemplation, of sacrifice, and of charity, a luminous word comes immediately to the lips: vocation!… This vocation, this call of love, makes itself felt in very diverse manners… But also the young Christian woman, remaining unmarried in spite of herself, who nevertheless trusts in the providence of the heavenly Father, recognizes in the vicissitudes of life the voice of the Master: “Magister adest et vocat te” (John 11:28); It is the master, and he is calling you! She responds, she renounces the beloved dream of her adolescence and her youth: to have a faithful companion in life, to form a family! And in the impossibility of marriage she recognizes her vocation; then, with a broken but submissive heart, she also gives her whole self to more noble and diverse good works.
The Catechism does not directly speak about this, but it touches implicitly on it, when it speaks about those single persons who are without a family by circumstances beyond their control, and who serve God and neighbor in an examplary fashion.
1658 We must also remember the great number of single persons who, because of the particular circumstances in which they have to live – often not of their choosing – are especially close to Jesus' heart and therefore deserve the special affection and active solicitude of the Church, especially of pastors. Many remain without a human family often due to conditions of poverty. Some live their situation in the spirit of the Beatitudes, serving God and neighbor in exemplary fashion. The doors of homes, the "domestic churches," and of the great family which is the Church must be open to all of them. "No one is without a family in this world: the Church is a home and family for everyone, especially those who 'labor and are heavy laden.'"
More articles on the single vocation