PRAYER WITH NON-CATHOLICS FORBIDDEN
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. MORTALIUM ANIMOS ENCYCLICAL OF POPE PIUS XI
ON RELIGIOUS UNITY
Are heretics and schismatics excommunicated? Yes; they have no part in the Communion of the Saints. Catechism of the Summa
If any ecclesiastic or layman shall go into the synagogue of the Jews or to the meeting-houses of the heretics to join in prayer with them, let them be deposed and deprived of communion. If any bishop or priest or deacon shall join in prayer with heretics, let him be suspended from communion. II Council of Constantinople
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! Ven. Pope Pius XI
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. Council of Carthage
No one shall pray in common with heretics and schismatics. Council of Laodicea
Since you are not of my religion, I ought not to join in prayer with you; but I will pray heartily for you: that God would enlighten you, bring you back to His Church, and dispose you for His mercy. Ven. Henry Walpole
I will not pray with you, nor shall you pray with me; neither will I say “Amen” to your prayers, nor shall you to mine! St. Margaret Clitherow
I refuse to pray with you, but I desire all Catholics to pray for me, and I mean Catholics of the Catholic Roman Church. Ven. George Haydock
We decree that those who give credence to the teachings of the heretics, as well as those who receive, defend or patronize them, are excommunicated. […] If anyone refuses to avoid such accomplices after they have been ostracized by the Church, let them also be excommunicated. IV Lateran Council
VISITING PROTESTANT CHURCHES
It is not right that one who has been condemned and cast out by the Apostolic See of Rome for his wrong opinions should be named with any kind of honour until he be received by her, having returned to her by a pious confession and orthodox faith. St. Maximus of Constantinople
You have no right to receive a heretic into your communion. Pope St. Simplicius
Therefore, receive into communion those who teach and think as I do; but refuse those who are otherwise minded, and hold them as strangers to God and to the Catholic Church. St. Gregory Nazianzen
In the celebration of the sacred mysteries, there should not be recited the names of those who have been separated from communion with the Catholic Church. III Council of Constantinople
Put away the evil one from among yourselves. I Corinthians 5:13
Cut off the decayed flesh, expel the mangy sheep from the fold, lest the whole house, the whole paste, the whole body, the whole flock, burn, perish, rot and die! St. Jerome
These men are Protestants; they are heretics! Have nothing to do with them! St. Anthony Mary Claret
I will live and die in the true Roman Catholic faith, which I and all antiquity have always professed; and I will by no means go to Protestant churches. Ven. Edmund Genings
It is not lawful to go to the Protestant church. St. John Rigby
Abhor schism, and adhere to the Catholic Church! St. Hippolytus of Rome