Throughout the history of the Church, many have claimed that the Blessed Mother has miraculously appeared to them. Some of these apparitions have been approved by the Church. Most have not been approved.
"Apparition" just means that a heavenly being—Christ, Mary, another saint, or an angel—makes himself known to human senses. The Bible contains descriptions of many such appartitions. The first apparitions were to Adam and Eve, when God walked with them in the cool of the garden. Then have a look at Exodus, when God appeared to Moses and spoke to him in the burning bush. Carry it through to the Annunciation, the Nativity, and the Resurrection. Look at Revelation, in which John describes his vision of the whole heavenly Jerusalem. The whole Bible is the transcript of one apparition after another.
Apparitions, outside of those described in Scripture, are considered "private revelation," and while some have been recognized by the Church, they do not belong to the deposit of faith. Catholics are not bound to believe Church-approved private revelations. The Church has never taught that we must believe in any apparition or that belief in any apparition was necessary for us to be saved.
The Catechism explains the role of private revelation as follows:
It is not their role to improve or complete Christ’s definitive revelation but to help live more fully by it in a certain period of history. Guided by the magisterium of the Church, the sensus fidelium knows how to discern and welcome in these revelations whatever constitutes an authentic call of Christ or his saints to the Church. Christian faith cannot accept "revelations" that claim to surpass or correct the revelation of which Christ is the fulfillment, as is the case in certain non-Christian religions and also in certain recent sects that base themselves on such "revelations." (Catechism of the Catholic Church 67)
How does the Church judge a claim of an apparition or other private revelation?
The following text taken from the University of Dayton’s website explains.
You can find this page at http://campus.udayton.edu/mary/resources/newsltr.html
The text is based on "Discerning the Miraculous: Norms for Judging Apparitions and Private Revelations" by Frederick Jelly, O.P. (1993); and "Canonical Considerations regarding Alleged Apparitions" by Fr. Michael Smith Foster, JCD (1995). It was abridged, combined and rearranged by Fr. Johann G. Roten, S.M.
Realism
During the past several years, the number of reported apparitions has been greatly increasing. While concentrating on Marian apparitions, our purpose here is to view all popular piety in the context of the total picture of Catholic faith, devotion, and discipleship. Our approach is to strike a happy medium between vain credulity and sterile skepticism. Perhaps we could label our position as one of critical or moderate realism.
Authentic Visions
Visionaries or seers behold an object not naturally visible to other persons. An authentic supernatural vision or apparition is different from illusions or hallucinations that result from pathological conditions or even diabolical intervention. An authentic vision is a charism -- gratia gratis data -- given to an individual or group for the spiritual good of others and/or for the Church as a whole.
The Church's Voice
On February 25, 1978, the Sacred Congregation for the Doctrine of the Faith (CDF) issued "Norms of the Congregation for Proceeding in Judging Alleged Apparitions and Revelations." Pope Paul VI had approved these norms the preceding day.
Although this document uses the term supernaturalitas, we believe that what is meant is something supernatural in the mode in which it has occurred rather than supernatural in its essence or substance.
The Bishop's Role
When an allegedly supernatural event has occurred, it is the responsibility of the local bishop to conduct an investigation, usually through a committee of experts.
Theologically and canonically, the bishop is entrusted with the role of "oversight" of the diocese. This role of "oversight" is based on the bishop's responsibility both for public worship and for the religious teaching which occurs in the diocese.
Norms for Discernment
1) The first norm for evaluating miraculous events is that there be moral certainty, or at least great probability, that something miraculous has occurred. The commission may interview the visionaries, call other witnesses, visit the site of the events.
2) The second norm deals with the personal qualities of the subjects who claim to have had the apparition; they must be mentally sound, honest, sincere, of upright conduct, obedient to ecclesiastical authorities, able to return to the normal practices of the faith (such as participation in communal worship, reception of the sacraments).
3) A third category deals with the content of the revelation or message: it must be theologically acceptable and morally sound and free of error.
4) The fourth positive criterion is that the apparition must result in positive spiritual assets which endure (prayer, conversion, increase of charity).
These four criteria may also be restated in a negative way. There must be no doubt that what is occurring is truly exceptional and beyond human explanation. There must be no doctrinal error attributed to God or to the Blessed Virgin Mary or some other saint. The third negative criterion is that there must be no hint of financial advantage to anyone connected with the apparitions, nor must any of the visionaries be accused of serious moral improprieties at the time when the visions were being received, nor may there be evidence of mental illness or psychopathic tendencies.
Verdicts
At the end of the investigative process, the committee may submit to the bishop(s) one of the following verdicts or conjectural judgements: constat de supernaturalitate (the event shows all the signs of being an authentic or a truly miraculous intervention from heaven); constat de non supernaturalitate (the alleged apparition is clearly not miraculous or there are not sufficient signs manifesting it to be so); non constat de supernaturalitate (it is not evident whether or not the alleged apparition is authentic).
The Bishop's decision regarding alleged apparitions usually does not attempt to interpret or give the spiritual significance of the events, nor to interpret the messages or identify the heavenly persons who may have appeared.
Public Worship
All such investigations are conducted by the bishop to determine whether public worship should continue to be held in those places. It could happen that the investigative committee could conclude that at this time it is able neither to make a clearly positive nor a clearly negative judgement. If the matter is still being investigated, the bishop could permit public worship, while at the same time continuing to be vigilant that the devotions do not wander into deviant directions.
Our Responsibility
As the bishops are entrusted with these responsibilities stemming from the nature of their office, so there are fundamental responsibilities on the part of the members of the diocese. First, they are to obey their bishops when the latter act as Christ's representatives (canon 212), that is, when they teach formally or establish binding discipline as pastors of a particular church. This obedience owed to the bishops in their capacity as leaders of particular churches is intended to promote the common good. Canon 753 also speaks of the "religious assent" owed to the bishops' teaching authority, which means a special quality of respect and gratitude, along with critical awareness and good will. Hence, there should be intelligent obedience to ecclesiastical authority in the matter of alleged apparitions.
Publications
A new era opened in the canonical regulations dealing with apparitions occurred in 1969. In that year, Pope Paul VI deleted certain canons of the Code of Canon Law (1917). These canons had specifically forbidden the publication of all books or pamphlets about new apparitions, revelations, visions, prophecies, and miracles, or which introduce new devotions, even though justified as private. Such prohibitions are not part of the Code of Canon Law (1983). So the many reports of Marian apparitions may in part be due to the new freedom to discuss freely and to report such occurrences to the media, without first submitting them to ecclesiastical approbation.
The Foundations of Our Faith
Our faith cannot rest on private revelations and apparitions. Even with properly approved apparitions, we must maintain a proper perspective -- viewing them as an assistance to nourish our faith in the central dogmas of the Incarnation, the Trinity and the Eucharist. In their 1973 pastoral letter, Behold Your Mother: Woman of Faith, the American bishops called authenticated appearances of Mary "providential happenings [which] serve as reminders of basic Christian themes: such as prayer, penance, and the necessity of the sacraments." (# 100).
