The vast majority of evangelical Protestants remain in such churches, once they become Christians. Also, quite a number of Roman Catholics and Eastern Orthodox convert and join evangelical Protestant churches, particularly if their faith was rather nominal to begin with.
But interestingly, some evangelical Protestants move in the opposite direction, and either join the Roman Catholic Church, or they join an Eastern Orthodox church. So, why do some evangelicals bail out on Protestantism, to become members of these other churches? When it comes to Roman Catholicism, is it not true that Protestants fought long and hard to try to reform Catholicism, only to find themselves outside of the church of Rome? When it comes to Eastern Orthodoxy…. well,… what is Eastern Orthodoxy, anyway?
Well, there are multiple reasons why some evangelical Protestants either “cross the Tiber” (a metaphorical way of saying that they become Roman Catholic…. the Tiber River cuts through the heart of the city of Rome), or “cross the Bosphorus” (a metaphorical way of saying that they become Eastern Orthodox…. the Bosphorus is a body of water that goes through Istanbul, Turkey, the traditional heart of the Eastern Orthodox world). One reason is that in both Roman Catholicism and Eastern Orthodoxy, the celebration of the Lord’s Supper is central to their corporate worship. Whereas, in much of Protestantism, the celebration of the Lord’s Supper often takes a back seat, when compared to the teaching of the Scriptures.
But perhaps one of the main reasons for leaving evangelical Protestantism is a disillusionment with how Protestants often handle the doctrine of sola scriptura, from the Latin, or “Scripture alone.”
The idea of sola scriptura assumes that Scripture, by itself, can be interpreted, without an authoritative magisterium, or teaching authority, like the Pope (Roman Catholic) or college of bishops (Eastern Orthodoxy). But when Protestants rely on the private interpretation of Scripture, confusion has often ensued. Protestant Christians, in the United States, have been often known to “vote with their feet,” once they run into perceived problems with a teaching pastor, who says something that does not line up with how they read the Bible.
You do not have that problem in either Roman Catholicism or Eastern Orthodoxy.
The “vote with your feet” syndrome, that commonly divides Protestant churches, can become quite weary for some Christians. When Protestants are unable to work through their differences, it can get rather tiresome.
So, on the other hand, it is pretty much a “package deal,” if and when you decide to join either the Roman Catholic or Eastern Orthodox communions. Both Roman Catholicism and Eastern Orthodoxy have their own authoritative magisteriums. Such conflicting understandings of teaching authority has created another whole set of problems, which would take a comprehensive look at church history, to fully digest.
Those “package deals” presented by both older communions have presented obstacles for those Protestants who have considered making the journey across the Tiber or the Bosphorus, but who end up not crossing one of those rivers (I would include myself in this latter category). For example, both Roman Catholicism and Eastern Orthodoxy hold to what is called the doctrine of the perpetual virginity of Mary. That is a big stumbling block that keeps many evangelical Protestants from seriously considering crossing “the” river.
If you want to learn more about why some Protestants look to Rome or Eastern Orthodoxy, these two short videos, respectively, help to explain why: