John MacArthur often has thoughtful and interesting things to say: just not, typically, on Catholicism. On this topic, he says things like:
While there are many errors in the teaching of the Catholic Church (for example its belief in the transubstantiation of the communion wafer and its view of Mary), two rise to the forefront and call for special attention: its denial of the doctrine of sola Scriptura and its denial of the biblical teaching on justification. To put it simply, because the Roman Catholic Church has refused to submit itself to the authority of God’s Word and to embrace the gospel of justification taught in Scripture, it has set itself apart from the true body of Christ.Of course, these doctrines are products of the Reformation, and even the Calvinist scholar and historian Alister McGrath has admitted that Protestant views on justification are a "theological novum," in that they weren't taught by anyone anywhere prior to the Reformation. In fact, no pre-Reformation Church -- the Coptics, the Eastern Orthodox, the Oriental Orthodox, or the Catholics -- believes either of these tenets.
So to put it simply, if you have to believe these two things to be part of the "true Body of Christ," His Body simply wasn't present on Earth from the time of His death until the Reformation.
But besides that, let's consider the obvious. Lots of Reformed preachers declare that Catholicism preaches a "Different Gospel" because She affirms the historic Christian view of justification, rather than the warped novelty. Based on this, a number of Protestants feel free to declare out-of-hand that Catholics aren't Christians. But let's assume that everyone on Earth was wrong about justification for a millennium and a half. Would this mean She was cut off from the Body of Christ?
It would certainly mean that She wasn't the Body of Christ, since the Bride of Christ contains no doctrinal errors. But She could still be a part of the Body, since the members of the Body are themselves individually imperfect. I'm at a consistent loss for why Calvinists (and that's who it usually is) can declare that Catholics aren't Christian because they don't toe the Reformed line on justification. Why not declare that Baptists aren't Christian because they don't toe the Reformed line on infant baptism? If the standard of Christianity becomes how closely one's Faith matches the writings and Biblical interpretations of a handful of sixteenth century Europeans (a pretty arbitrary standard, in my estimate), why not say things like "To put it simply, because the Baptist Church has refused to submit itself to the authority of God’s Word and to embrace the gospel of infant baptism taught in Scripture, it has set itself apart from the true body of Christ."
Who died and left self-righteous Calvinists like John MacArthur in charge of deciding membership in the Body of Christ? Certainly, not Christ (Matthew 16:17-19); He died and left someone else in charge, if memory serves. In fairness to Calvinists, MacArthur's view isn't the only one; but it's certainly the view which was popularly held by many American Calvinists for the last few centuries.