What I didn't know until Ben Wright buzzed it was that Protestants had been invited to this Council. Which is not anything particularly new, the Catholics had invited the Lutherans to the Council of Trent (though not the Reformed, for that discourse you've got to read Calvin's letters to Sadoleto). And just as the Lutherans had in the 16th century, so the Presbyterians of the 1800s declined the Pope's invitation. Speaking for the PCUSA, Charles Hodge wrote the response, the entirety of which can be found here:
But although we do not decline your invitation because we are either heretics or schismatics, we are nevertheless debarred from accepting it, because we still hold with ever increasing confidence those principles for which our fathers were excommunicated and pronounced accursed by the Council of Trent, which represented, and still represents, the Church over which you preside.