When someone claims to have received a private revelation from God or a saint, the Church investigates the situation to determine if it is in accord with the faith.
Recently the Dicastery for the Doctrine of the Faith released updated guidelines that helps explain the process and what is actually "approved" in the investigation:
As a rule, these potential conclusions do not include the possibility of declaring that the phenomenon under discernment is of supernatural origin—that is, affirming with moral certainty that it originates from a decision willed by God in a direct way. Instead, as Pope Benedict XVI explained, granting a Nihil obstat simply indicates that the faithful “are authorized to give [the phenomenon] their adhesion in a prudent manner.” Since a Nihil obstat does not declare the events in question to be supernatural, it becomes even more apparent—as Pope Benedict XVI also said—how the phenomenon is only “a help which is proffered, but its use is not obligatory.”
Basically, individuals are free to believe in the alleged apparitions and to follow any devotions connected to it.
Holiness of the visionary
The investigation of an alleged apparition will dig deeper into the holiness of the visionary as well:
The credibility and good reputation of the persons who claim to be recipients of supernatural events or to be directly involved in them, as well as the reputation of the witnesses who have been heard. In particular, one should consider the mental equilibrium, honesty and moral uprightness, sincerity, humility, and habitual docility toward ecclesiastical authority, willingness to cooperate with it, and promotion of a spirit of authentic ecclesial communion;
The fruits of the Christian life, including a spirit of prayer, conversions, vocations to the priesthood and religious life, acts of charity, as well as sound devotion and abundant and constant spiritual fruits. The contribution of these fruits to the growth of ecclesial communion is to be evaluated.
However, even if the visionary is believed to be of a sane mind, it does not mean they are a saint.
The Catechism of the Catholic Church offers a brief explanation behind the Church’s main criteria during the canonization process:
By canonizing some of the faithful, i.e., by solemnly proclaiming that they practiced heroic virtue and lived in fidelity to God’s grace, the Church recognizes the power of the Spirit of holiness within her and sustains the hope of believers by proposing the saints to them as models and intercessors.
It is possible for a vision to be approved by the Church, while at the same time, the visionary is never canonized.
The visionary will need to show evidence of "heroic virtue" in order to be considered for the canonization process.
Not all visionaries led lives of heroic virtue, even though God may have granted them a private revelation.
