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As Peter Leithart has observed, the root cause of the split in the Church at the time of the Reformation was the Church’s tolerance of idolatrous practices and ways of thinking about salvation.
I'm curious what exactly he is referring to here? Is it the Mass? If so, then that is still alive and well, thank you, and the Catholic Church is not (from this critical perspective) one whit less "idolatrous" than it was in 1517.
The problem then becomes: "how does one accept as brothers and sisters in Christ a communion that (so the criticism goes) enshrines rank, gross idolatry at the very center of its worship?" It makes no sense to take a middle position of "sure, Catholics are Christians" but then hold that they commit idolatry every Sunday.
The other thing about the sacrifice of the Mass is that it has considerable patristic support.
I'm assuming that the idolatry referred to is the Mass, but say it was the Catholic doctrine of Mary instead. If so, then again, our Mariology is considerably more robust now than it was then, with the Immaculate Conception and the Assumption now ex cathedra binding doctrines.
So you would have to say (from a Reformed perspective) that Catholics are far worse on both scores now than they were then. Even papal infallibility is far more defined now than then (even conciliar infallibility -- in the fully Catholic, papal sense -- saw significant development in Vatican II).
As you can see, I'm trying to trap the ecumenical Reformed position on the horns of a dilemma. I would contend that anti-Catholic Protestantism (much as I loathe and detest it) is actually more consistent than an ecumenical Protestant position that continues to regard the Mass as idolatry and blasphemy (I've pressed Lutherans on this, too, and they seem to have no ready reply). The anti-Catholics call a spade a spade. That's how they view Catholics and so, consistently, they read them out of Christianity altogether.
To do ecumenism with Catholics, and to be halfway consistent, you will have to find a way to "tolerate" the Mass as the Mass, and to not have the schizoid view described above. The Mass would have to be, it seems to me, regarded as not essentially different in kind or degree of error than other Protestant forms of worship that differ from Reformed (say, Pentecostal or Mennonite).
On a more positive note: I agree wholeheartedly with your remarks about the definition of "the gospel". Indeed, I have been making virtually an identical argument for many years now, both as a Protestant and as a Catholic. It's refreshing to see some Reformed Christians agreeing and refusing to redefine the gospel as TULIP.