Mormons believe that the Catholic Church was once valid, but became invalid. They call this the “Great Apostasy.”
When asked to justify this claim, they resort to the typical claims which are familiar to anyone who has studied or challenged typical anti-Catholics. They claim that the Church “gradually” fell into apostasy because of a preponderance of sinful or absurd actions by various Catholics (including Popes - expect to hear about Pope Formosus and the (shameful) Cadaver Synod).
But, actually (and ironically) they set the bar very high. The Catholic Church also has a definition of general apostasy, and it is nowhere near as demanding as the Mormons teach. The Catholic Church regards Herself as apostate if She teaches JUST ONE doctrine that is not part of the Deposit of Faith. She may not teach ANY new doctrine, and She may not do away with ANY existing doctrine, and she may not change ANY doctrine whatsoever. Catholics do not need any sort of preponderance of evidence to declare the Catholic Church apostate - we just need ONE such example.
If the Catholic Church is, indeed, apostate, it was not a gradual process - it was an instant event. It happened the very moment that the Church invented a new doctrine, or abolished an existing doctrine, or changed a doctrine.
Ask them to define when this specific event occurred.
If they can do it, then you (and I) should convert to Mormonism. But they can’t do it, because it has never happened. I know this because I once opposed the Church’s claims, and I diligently tried to disprove this claim, but I was unable. This was a significant factor in my conversion to the Catholic faith.
