My question is, if V2 was not the reason for the decline, then what is the main cause.
That’s a good question. One starting-point to consider when answering it is the Eastern Rite Byzantine Catholic world before and after Vatican II.
First, the move to vernacular was heavily-encouraged at Vatican II by an Eastern Rite Catholic bishop. For him, there was no problem using vernacular in the liturgy - since the Eastern Rites have used vernacular for a long time prior to Vatican II.
So there’s a simple change: moving from Latin-only to vernacular. Was that evil? No - it couldn’t be because Eastern churches used vernacular and they didn’t have a decline (in fact they are increasing). It only happened in the West.
How big of a change was the move from Latin to vernacular?
Very simply - it was a massive, earth-shattering change.
It doesn’t seem like a big deal now, but it really was.
What happens when a big organization makes a massive, cultural change?
Without very tight controls, things can easily get misinterpreted and misrepresented.
Ok, that’s just one change - Latin to vernacular. It affected the whole world.
So, was Vatican II responsible for the decline?
We could say “yes” because Vatican II authorized vernacular (although the text says that “Latin must be retained”).
But that does not make sense because it would mean that the Western Church would never be able to use vernacular in the liturgy, although the Eastern Rites could without a problem. Why? Are Western Catholics incapable of worshipping God in the vernacular language without running off into heresy and apostasy?
Or, did Western Catholics love the Latin language so much that once it was lost they stopped going to Church?
Let me put it this way … there’s a lot more to the story. The decline that we’re facing now in many countries (not all) had started decades before Vatican II in Europe. Dissent, modernism, lax Catholics, declining Mass attendance … all had been in place in Europe for a while. True, not as bad as it is now, but it was there. Also true, nobody would consider going to Communion without Confession. They were taught correctly. Instead, they just dropped out of the parish.
We could look at the life of St. John Vianney in a fairly typical rural community in France in the mid-1800s. What did he find among the Catholics there? Drunkenness and carousing, a sacrilegious spirit and a very poor attendance at Mass.
Ironically, the way he fixed it was with personal holiness, solid preaching and hours in the Confessional – exactly the things we need today. But the point is – there’s no proof that the documents of Vatican II directly caused the decline or that the changes intended by Vatican II were evil in that way. There’s nothing in the documents that should have caused people to stop going to Confession and stop going to Mass.