When someone joins Opus Dei he does so by means of a verbal contract between himself and Opus Dei in which he agrees to try to live the spirit of Opus Dei, to live the plan of life described above, to carry out an active apostolate, and to help the apostolic activities of the Prelature, according to his talents and availability. Opus Dei, for its part, agrees to provide the spiritual direction and the assistance needed to do so.
Every March 19 members must renew the contract in the privacy of their own prayer and then notify Opus Dei that they have done so. Failure to do either of these two things without a serious reason automatically results in the member no longer being in Opus Dei. It’s that easy to leave, and that is precisely how I left.
After 5 years a supernumerary may make the “fidelity,” which is a life-long commitment to remain in Opus Dei. It is not a vow, but a promise, based on his honor as a Christian. After making the fidelity the supernumerary no longer has to make the yearly renewals. Supernumeraries are never required to make the fidelity.
Numeraries and associates must make the fidelity after 5 years, or become a supernumerary or leave the Work.
All Christians are required to live the virtue of chastity according to their state in life. Numeraries and associates, being single, live chastity as a single person. Supernumeraries who are married live chastity according to their state in life. There are no vows involved.
As St. Josemaría used to say, “We are interested in virtue, not vows.”
This is one of the many things that distinguish members of Opus Dei from members of religious orders. If you were to strike a numerary in anger, you would be guilty only of assault. If you were to strike a vowed Religious, in addition to assault you would be guilty of a sacrilege.
Another practical effect of the fact that members of Opus Dei don’t take vows is that there is no need for the involvement of the Holy See when a member of the Work leaves, even if they’ve made the fidelity. There are no vows to be released from. This is not to say that a member could abandon his vocation to Opus Dei in a cavalier manner without it being a mortal sin, at least objectively speaking. But if it were a sin, then the sin would be due to the serious nature of the obligation he was leaving behind, i.e., the obligation all Christians have to follow the will of God in their vocation, not because of any vows.
A reader pointed out to me that, philosophically speaking, if there is no identifiable difference between two things then they are, in fact, the same. His point was that while we don’t call them vows, members of Opus Dei do make an explicit “personal commitment” to live the virtues of poverty, chastity, and obedience according to their state in life, and that this “personal commitment” is really the same thing as a vow. While I agree that “if it looks like a duck, smells like a duck and swims like a duck, it must be a duck,” I don’t agree that it applies here because of the clear differences mentioned above and because canon law clearly defines what a vow is, and for it to be a vow, it must be called a vow.
Added April 8, 2016:
The need for the Holy See to release a person from a religious vow, as contrasted with the lack of such involvement when a person leaves the Work, is really significant here. For example, if a priest, nun, or Religious brother attempts to marry without first being formally released by the Holy See from his or her vows, then the marriage would not be valid. But if a person were to simply walk away from the Work, even after making the fidelity, there would be no obstacle to a valid and holy marriage. This alone makes clear the real and important distinction between a vow and the promises made when joining Opus Dei.