Pope Leo X, Fifth Lateran Council, Session 8, ex cathedra: "And since truth cannot contradict truth, we define that every statement contrary to the enlightened truth of the faith is totally false and we strictly forbid teaching otherwise to be permitted. We decree that all those who cling to erroneous statements of this kind, thus sowing heresies which are wholly condemned, should be avoided in every way and punished as detestable and odious heretics and infidels who are undermining the Catholic faith."
The Pope Leo X just said infallibly that all heretics should be avoided in every way. Note that you can only know that someone is a heretic if you yourself have obtained this knowledge of the person in question.
III Council of Constantinople, 680-681: “If any ecclesiastic or layman shall go into the synagogue of the Jews or the meetinghouses of the heretics to join in prayer with them, let them be deposed and deprived of communion [excommunicated]. If any bishop or priest or deacon shall join in prayer with heretics, let him be suspended from communion [excommunicated].”
The Third Council of Constantinople just defined infallibly that any person who prays in communion with heretics are to be excommunicated and refused communion for praying with other heretics. Now let’s look at some other quotes:
Council of Laodicea, 4th century, (#Canon 6): "No one shall pray in common with heretics and schismatics… It is not permitted to heretics to enter the house of God while they continue in heresy."
Council of Carthage: "One must neither pray nor sing psalms with heretics, and whoever shall communicate with those who are cut off from the communion of the Church, whether clergy or layman: let him be excommunicated."
Pope Pius IX, Sept. 16, 1864, letter to the English Episcopate (CH 254): "That Christians and ecclesiastics should pray for Christian unity under the direction of heretics and, what is worse, according to an intention which is radically impregnated and vitiated with heresy, is absolutely impossible to tolerate!"
1917 Code of Canon Law, Canon 823: "Mass may not be said in churches of heretics or schismatics, even though they were in the past properly consecrated or blessed."
Pope Pius XI, Mortalium animos (# 10): "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. To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it."
For people then to claim (in spite of all the quotations above saying otherwise), that an assembly presided over by heretics or an assembly that prays in communion with other heretics, to somehow be the Church of God or the Church of Catholics, is simply to deny God's revealed infallible truth.
