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 … Read the rest