What is the number of Catholics who just sort of “deconvert” - period? I’m guessing that that is the biggest number of them all.
As recently as five years ago, the largest religious body in the US was Catholic and the second largest ex-Catholic.
All the mainstream Protestant denominations are losing members; the Episcopalians are determinedly self destructing before our eyes and the Lutherans and the others are slowly fading.
Individual megachurches are coming and going. The Catholics would be slowly growing if it were not for the immigrants. But Catholicism world wide is growing fast, not just here.
I don’t know the answer to your question but just anecdotally there are many re-converts. The difference is that re-converts don’t have to go through the RCIA process and they aren’t re-accepted into the Catholic church, they just go to confession and start living their faith again. They would never be considered NEW Catholics or converts. They would not be counted as such either.