When I lecture or debate on whether it is possible to “prove” the resurrection of Jesus on historical grounds, I talk about how — whether you believe in the resurrection or not — almost certainly the reason the disciples originally *believed* Jesus had been raised is that one or more of them had a vision of him after he died. (Believers would say their “vision” was something they actually saw; non-believers would say they were mistaken for one reason or another, or they imagined it, etc — that it was a hallucination of some kind).
But it is often noted that in the New Testament, after his death Jesus appears not only to individuals (Peter, Paul, and Mary, for example) (!) but to groups (the “twelve,” the “apostles” and “500 people” at one time, according to 1 Cor. 15:5-8). But how could *that* be possible? One person might mistake something she saw for a person, or dream they saw someone, or whatever. But *groups* of people? How can historians possibly explain “group visions” of a person unless the person was really there? You can’t plausibly argue for “group HALLUCINATIONS” can you? It seems counter-intuitive: aren’t hallucinations by definition the inner workings of a person’s mind?
So how does that work? My answer is, I don’t know: my guess is that one person says they see something and others are persuaded and “see” it too, or later “remember” that they too saw it; but, it’s just a guess. In any event, as odd as it might seem, group hallucinations are a well-documented phenomenon. Or at least they are explicitly AFFIRMED precisely by the apologists who complain that I can’t explain the appearances of Jesus to groups because groups cannot have hallucinations. That is to say, even though these apologists say that “it’s not possible” when it comes to Jesus, they think that it does happen in other instances. Most specifically, in the instance of Jesus’ MOTHER.
These apologists (the ones I’m talking about) are Protestants. They don’t believe Mary appears to anyone, let alone groups. How then do they explain that groups *claim* they have seen her? Well, apart from saying they don’t really see her even though they claim they do, they don’t have anything to say. Why is it different with Jesus?
So are group hallucinations possible? Here is what I say about it in my book How Jesus Became God.
The Blessed Virgin Mary & Group Hallucination
René Laurentin is a modern-day Catholic theologian and expert on modern apparitions, who has written many books on the topic. He has a degree in philosophy from the Sorbonne in Paris, and two PhDs, one in theology and one in literature. He is not your average intellect. He deeply and sincerely believes that Mary – the mother of Jesus who died 2000 years ago — has appeared to people in the modern world and that she continues to do so. Here I give just two examples from his writings.
In Cua, Betania, in Venezuela, a woman named Maria Esperanza Medrano de Bianchin received peculiar spiritual powers: she could tell the future, levitate, and heal the sick. The Virgin Mary appeared to her several times, starting in March 1976. The most striking occurrence involved lots of other people, on March 25, 1984. After the Catholic mass that morning, a number of people went to enjoy some time outdoors near the local waterfall, when the Virgin Mary appeared above it.
A Series of Visions
This began a series of visions. Mary came and went, often visible for five minutes or so, the last time for half an hour. Among the observers were doctors, psychologists, psychiatrists, engineers, and lawyers. People over the weeks to come started picnicking there. At times up to a thousand people observed Mary there, bathed in light and accompanied by the smell of roses. This continued until 1988. Later a Jesuit priest, Monsignor Pio Ricardo, who was a professor of psychology at the Central University of Caracas, interviewed 490 people who claimed to have seen Mary there. They convinced him.
A second example comes from Cairo, Egypt in 1986, at a Coptic church. Mary had appeared a number of times between 1983 and 1986. Once she appeared on the roof, four Coptic bishops arrived to authenticate the vision. They did indeed see her. At other times she was seen by (non-Christian, obviously) Muslims. In some instances, she was actually photographed. Laurentin indicates that he actually has a photograph of a similar apparition from another Coptic suburb, from 1968.
People BELIEVED Mary was Appearing: Group Hallucinations
My point is not that Mary really is appearing in these times and places. But people deeply believe she is. And it is not just illiterate peasants, but highly educated people. Terrifically anecdotal collections of Mary’s visions can be found in numerous books, such as Janice Connell’s Meetings with Mary: Visions of the Blessed Mother. 
Connell provides sixteen chapters detailing visions of Mary, from a believer’s perspective, from the nineteenth and twentieth centuries, as these are documented from Lourdes, Fatima, Garabandal, Medjugorje, and so on. There is, for example, the “cosmic miracle of the sun” that took place at Fatima on October 13, 1917. We are told the sun was seen to spin wildly and tumble down to earth before stopping and returning to its normal position, radiating indescribably beautiful colors. The miracle was seen and attested to by over 50,000 people.
Do such miracles happen? Believers say yes, unbelievers say no.
But it is striking and worth noting that typically believers in one religious tradition often insist on the “evidence” for the miracles that support their views and completely discount the “evidence” for miracles attested in some other religious tradition, even though, at the end of the day, it is the same kind of evidence (for example, eyewitness testimony) and may be of even greater abundance.
Protestant apologists who are interested in “proving” that Jesus was raised from the dead rarely show any interest in applying their finely-honed historical talents to the exalted Blessed Virgin Mary.
 See, for example, René Laurentin, The Apparitions of the Blessed Virgin Mary Today (Dublin: Veritas, 1990; French original 1988). The examples that I give below are all drawn from this book.
 New York: Ballantine Books, 1995.