Schism

1. Catholic polemical theology stresses the grave sin of schism. But that's circular because schism in Catholic usage is basically a made-up category. And don't quote me NT passages that use the word "schism" in Greek, because that's a semantic fallacy. In ecclesiastical usage, "schism" is a technical term. NT Greek usage doesn't have that specialized meaning.

The NT does have two related categories: heresy and apostasy. The clearest NT example of schism are the heretics who broke away from St. John's congregations (1 John). But, of course, evangelicals don't think the Catholic church is comparable to John's congregations. If anything, it's the other way around. The Catholic church broke away from NT exemplars. 

2. Here's another problem with the traditional Catholic allegation that Protestant denominations are schismatic. Schismatic in relation to what? To the church of Rome? A basic problem with that comparison is that the church of Rome keeps reinventing itself. Even though Protestants broke away from the 16C church of Rome, that no longer exists. It isn't possible to reunite with the 16C church of Rome, even if that was desirable. The 16C church of Rome is radically different from the 21C church of Rome. The 1C church of Rome is very different from the 6C church of Rome. How can you be schismatic in relation to a "parent" denomination that's constantly changing? The church of Rome keeps mutating into something else. 

Of course, Catholic apologists insist that there's fundamental continuity, but that's only convincing to Catholic apologists and like-minded Catholics.