The Church says that apparitions of Mary have happened since Biblical times. However, you can disbelieve in them and still be a good Catholic. They are not part of the Catholic faith. The Church merely gives permission to believe in an apparition not a command. If an apparition contradicts Catholic doctrine, the Church forbids belief in the apparition by way of command.  It may give a direct command but if there is no direct command it is still covered by the law that out of respect say for Jesus and Mary you cannot be following appearances that are not from them.  Part of the condemnation of Spiritualism/spiritism in the Bible is about how having a relationship with a spirit you think is your deceased loved one and getting messages cheats you and insults them if it is not real.

Even the visionary is not bound to believe in their apparition. The Church says apparitions are only permitted to be believed in if they teach correct doctrine and if they don’t command anything strange or dangerous.

The Lourdes apparitions of 1858 are dubious for the story goes that Mary gave St Bernadette a secret to be kept for herself. She never told the Church investigators what it was. Thus the Church permission to decree the acceptability of belief in the apparition was invalid for they did not have all the information.

Mary supposedly appeared at a dirty disease ridden dump and had Bernadette eating plants there and thus risking her health. A spring appeared and Mary urged Bernadette to tell the crowd to use it. Bernadette tried to drink the water and stuffed soggy earth into her mouth. An apparition that nobody is bound to believe in has no right to command such harmful things. If you start saying, “Our Lady protected them from any harm”, you are really saying that people should take terrible risks for themselves and others on no other basis but faith.

The Knock apparition where Mary, St Joseph and St John seem to have appeared in 1879 with the Lamb of God on an altar seems very convincing on the face of it. But the Church did it's best to present the world with an altered version of what really happened.

Then there is Medjugorje where Mary has been appearing since 1981. Pilgrims go there and stare into the sun. Why no warning from Mary at Medjugorje not to look into sun? Does she not care about the huge number of cases of eye damage?

Medjugorje also presents her as having her messages distributed without being checked by Church authority. As the bishops for example know religion better than ordinary Catholics, Mary would at least honour that. Also, it is Catholic teaching that Jesus left his teaching authority with the bishops and the pope. Thus he cannot send an apparition that will contradict or undermine the authority of the bishop.

The messages being promoted by her are in disobedience to the Vatican.

The Vatican, The Congregation for the Doctrine of the Faith, November 1996

Regarding the circulation of texts of alleged private revelations, the Congregation states:

The Interpretation given by some individuals to a Decision approved by Paul VI on 14 October 1966 and promulgated on 15 November of that year, in virtue of which writings and messages resulting from alleged revelations could be freely circulated in the Church, is absolutely groundless. This decision actually referred to the "abolition of the Index of Forbidden Books" and determined that --- after the relevant censures were lifted --- the moral obligation still remained of not circulating or reading those writings which endanger faith and morals.

In should be recalled however that with regard to the circulation of texts of alleged private revelations, canon 623 #1 of the current Code remains in force: "the Pastors of the Church have the … right to demand that writings to be published by the Christian faithful which touch upon faith or morals be submitted to their judgment".

Alleged supernatural revelations and writings concerning them are submitted in first instance to the judgment of the diocesan Bishop, and, in particular cases, to the judgment of the Episcopal Conference and the Congregation for the Doctrine of the Faith.

Any apparition that breaks this rule, and the Medjugorje Virgin commands that her messages be distributed as they are received, is not a Catholic apparition or concerned about Catholic orthodoxy. The bishops are the official Catholic teachers not apparitions. The vast majority of modern visions break the rule and so are themselves disobedient apparitions.

The Catholic Church says that if you don't believe in miracles then you are saying that the wonderful testimonies from the most reliable of people that they happen are to be dismissed. The Church says that if people are that lacking in credibility, we can believe nothing they say. You can reply that you are not saying they must be dismissed or ignored. Trust means that you expect to hear sane things and decent things from decent people.