I didn't want to write that "Calvinists need to quit acting as if the PCUSA liberals aren't Calvinists" because, well, I'm the first one to say that liberal heretics from the Catholic tradition aren't Catholics. But my attempt to avoid saying that was just clumsy and kind of wound up saying it anyway. So let me put it this way:
Calvinists need to recognize that Presbyterian liberals are Calvinist heretics in the same way that social-justice Catholics are Catholic heretics. They aren't good Calvinists or Catholics; in a very real way, they aren't really even Christians. But the churches which they've apostasized from are Calvinist, and Catholic, respectively. The Calvinist origin of the PCUSA is as relevant as the Catholic origin of the social-justice "Catholics." If Calvinist denomination after denomination falls to liberalism, instead of merely escaping into ever more fragmented denominations, Calvinists need to confront why this happens, not just dismiss the "heretics" as not really Calvinist. Their reasons for heresy are Calvinist reasons.
Which takes me to why I believe the papal-episcopal polity of Catholicism is superior to the congregational-coalescence polity of Presbyterians. Catholics are obligated not to divorce themselves from the institution when it goes bad, but to wrest it back onto the right course. Presbyterian congregations just divorce themselves from the larger community. And God hates divorce. Not only as it expresses itself in the severence of the institution of marriage, which is a living symbol of the relationship between God and his church, but as it expresses itself in all forms of the severence of any holy bond. Any bond which can be severed isn't very holy, so what does it say about the bonds among Presbyterians that they can be severed?
I think it says that we don’t take our denomination name as seriously as Catholics that Protestant bodies split. Some Protestants make fun of an allegience to denomination and call it “Churchianity” to say that the focus on Christ is lost and it becomes a “My church is better than your church” sort of thing. They often view Catholics this way.
Remember also that the Catholics split and became Protestants. They were all Catholics before and then the Catholics decided they didn’t want to reform or compromise when called to do so by their own priests. Catholics divorced too rather than reconcile or work through the problem. The history of it can be debated — probably not fruitfully by me — but it was all Catholics that split hundreds of years ago. They couldn’t work through Luther’s (justified) call for reform.
I’m glad you realized the argument you made could be reversed though. Think of it in terms of Sampson and all the other era’s in the Bible when Israel turned from God. Their churches were apostate and the people worshipped foreign Gods. They couldn’t even remember their own traditions. The people wept when they heard Ezra read the law, maybe because they hadn’t understood it before or heard it. My point is that there were the saved even in the apostate times — even when the church was weak or non-existant.