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!”
