So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it. Mortalium-Animos, Pope Pius XI
All who enter the church of God and hear the Holy Scriptures, but do not communicate with the people in prayers, or who turn away, by reason of some disorder, from the holy partaking of the Eucharist, are to be cast out of the Church, until, after they shall have made confession, and having brought forth the fruits of penance, and made earnest entreaty, they shall have obtained forgiveness; and it is unlawful to communicate with excommunicated persons, or to assemble in private houses and pray with those who do not pray in the Church; or to receive in one Church those who do not assemble with another Church. And, if any one of the bishops, presbyters, or deacons, or any one in the Canon shall be found communicating with excommunicated persons, let him also be excommunicated, as one who brings confusion on the order of the Church.
Eh, don’t bother. I’ve already been told that “circumstances” have changed, so this no longer applies.
Darn tooting things have changed. It’s like two different religions.