Except for the most immature, and uneducated person, I've never met a "Calvinist" who believed this. Calvin himself, and even the most hyper-Calvinist Puritans, studied and valued the Church Fathers, Augustine, Bernard Clairveax, Hillary of St. Vincent, etc., and never suggested Hell had prevailed before the 16th Century. Of course God worked in and through the only church organization allowed to exist before the Reformation.
Calvinism too emphasizes grace most of all...saying we are not saved by our cooperation with God's mercy, rather He does all the saving...from start to finish. If Rodney Beason was a Pharisee as an Orthodox Presbyterian, it was because he didn't know Calvin and Calvinism enough, which puts the highest priority in the Word of God, by grace alone, through faith alone by Christ alone.
For every evangelical who becomes Roman Catholic, I'd bet there are 10 Roman Catholics in evangelical churches...including the OPC.
You wrote: “For every evangelical who becomes Roman Catholic, I’d bet there are 10 Roman Catholics in evangelical churches...including the OPC.”
I think you’re right. The real difference, however, is not the number. It’s the quality. The former Catholics you have left without ever learning their faith. I have seen this first hand, and read about it many times as well. The former Protestants, on the other hand, were much more likely to know their former faith, and much more likely to suffer loss (jobs, friendships, strained family relationships, etc.) when they converted.