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And with Medjugorje.

I can asure you that majority of Novus Ordo catholics in Croatia and Bosnia and Herzegovina are still believe in apparitions. 
There is radio station there in Medjugorje that promotes this. The wife of one of my first cousin used to work there illegaly. They didn't even employ her, payed insurance, etc. I was searching for a picture of Sacred Heart of Jesus, barely finding small one. But there are many "Divine Mercy" and "Our Lady of Medjugorje" images. It was in Ljubuški, a town near the border. They asked me if I need the receipt, they usually don't give you to avoid taxes. I was surprised because they are near the church. They built new modernist ugly church for their "praise and worship" charismatics. I didn't like that nuns and friars received Holy Communion mostly in the hand, but most of the laity still receive it on tongue. That was at least until 2019.-2020. 
When I was still in Novus ordo religion, I believe in Medjugorje, went there more than twice. I didn't meet visionaries. 
There is a great book in Croatian, from fr. Nikola Bulat "Istina če vas osloboditi", in eng. "The Truth will set you free" that debunks all hoax in Medjugorje. There are many citations from the diaries, statements from two bishops that opposed it, disobedience of Franciscian friars, etc. Most of my relatives and family still are deluded...

On topic, modernists can use good things for evil. They used Fatima site or church for intereligious meeting or even hindu worship. I read whole diary, there are traditional elements like doing penance, confession, adoration in front the Blessed Sacrament. But there is some questionable remarks, statements. We should trust more the condemnations rather than approval. Sadly cardinal Ottaviani also succuмb to progressivism, like archbishop Fulton Sheen.

It's not necessary, we have many older aporoved and more safe devotions that are linked with Mercy of Almighty God. So many Catholics never heard of Our Lady of Good Success, or Breton stigmatist Marie Julie Jahenny and Purple Scapular. 

"Oh such and such was cured cancer by xyz devotion it must be from God". No if the devotion is contrary to Catholic doctrine it's from the devil meaning the miracle was no miracle at all and God allowed it to happen to keep people in the dark, as for why, that's up to God and not anyone else's business.

There's probably no disease the devil can't cure with the knowledge of his angelic intellect ... if God permits it of course.  Demons know all the causes of and solutions to any health problem.  Heck they could even repair physical damage like severed spinal cords (to reverse paralysis).  Just because we don't have the knowledge or the tech doesn't mean it can't be done, since anything that has a physical cause can be physically fixed given the knowhow.

That's also why I disagreed with Bishop Williamson on those alleged Eucharistic miracles.  Even IF it were proven that the bread was replaced by some heart tissue (which it wasn't with those a some scientists believed they were molds, and a similar case in the US was just recently debunked as caused by mold ... with the same sequence of events as the Polish ones, where a host was dropped onto the ground, picked up and placed in the little water container.  After contact with the ground it had picked up some red mold spores that then grew in the water, as mold loves moisture). ... but even IF it were proven that, yes, this is living heart tissue, the devil could easily take away a bit of the bread and bring some human heart tissue to replace it with ... perhaps swooping in during a heart surgery or something, and therefore simulating the miracle.  Then even if the tissue remained "alive" for an extraordinary period of time, again, who's to say the demons don't know how to keep it animated, or just can't keep swapping it out?  Childsplay for them, if God doesn't prevent it.

So the FIRST TEST of any purported miracle or private revelation is the theological assessment.  If there's anything contrary to Catholic doctrine, it's immediately rejected as false (either just human fraud or diabolical in nature), or there's something very unbecoming in the behavior or moral life of the "visionary" ... regardless of how "convincing" it may appear to be.  No further invetigation is needed other than if someone were trying to expose human fraud for sure to prevent people from being deceived by it.

So with the NO Eucharistic Miracles ...

I've concluded that the NOM is displeasing to God and almost certainly invalid.  If that's correct, then what would the devil want more than to fool people into attending a Mass that's either invalid or valid but offensive to God?  So, hmmm. he might think, what's the best way to do that.  Right!  Fake some Eucharistic miracle to make people believe it's pleasing to God or at least not displeasing, and then certainly valid.  Therefore, I conclude that it's fake.  But someone with less conviction might use it to second-guess the initial conclusion and start wavering:  "Hmmm, maybe I'm wrong and the NO is valid and isn't all that displeasing to God."  That's precisely the devil's goal here.