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If you are one, then

We furthermore forbid any lay person to engage in dispute, either private or public, concerning the Catholic Faith. Whosoever shall act contrary to this decree, let him be bound in the fetters of excommunication. — Pope Alexander IV (1254-1261) in “Sextus Decretalium”, Lib. V, c. ii:

Do not converse with heretics even for the sake of defending the faith, for fear lest their words instil their poison in your mind. Bl. Isaias Boner of Krakow (Polish, Augustinian priest, theologian, professor of Scripture, d. 1471)

Thus, the Church forbids the faithful to communicate with those unbelievers who have forsaken the faith by corrupting it, such as heretics, or by renouncing it, such as apostates.

“the Church forbids the faithful to communicate with those unbelievers who have forsaken the faith they once received, either by corrupting the faith, as heretics, or by entirely renouncing the faith, as apostates, because the Church pronounces sentence of excommunication on both.” St. Thomas Aquinas, Summa Theologica

Is it permitted for Christians to be present at, or to take part in, conventions, gatherings, meetings, or societies of non-Catholics which aim to associate together under a single agreement everyone who, in any way, lays claim to the name of Christian? In the negative! - (Pope Pius XI, Mortalium Animos)

How does a Catholic sin against faith? A Catholic sins against Faith by Apostasy, heresy, indifferentism and by taking part in non-Catholic worship.” (Catechism of the Council of Trent, and the Baltimore Catechism)

“It is not permitted at all for the faithful to assist in any active manner at or to have any part in the worship of non-Catholics.” 1917 Code of Canon Law [Canon 1258]

“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 Bishops or Priest or Deacon shall join in prayer with heretics, let him be suspended from Communion” - III Council of Constantinople.

“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.

Rome, Italy, Feb 19, 2010 / 02:03 pm (CNA).- The president of the Pontifical Council for Promoting Christian Unity, Cardinal Walter Kasper, announced this week that Pope Benedict XVI will visit the Evangelical Lutheran Church located in Rome on March 14 for an ecumenical celebration.

“He is a heretic who deviates from any article of faith.” The American Textbook of Popery, p 143, (quoting from the “Directory for the Inquisitors”).

“The intolerance of the Church toward error, the natural position of one who is the custodian of truth, her only reasonable attitude makes her forbid her children to read or to listen to heretical controversy, or to endeavor to discover religious truths by examining both sides of the question.” (Stapleton, Explanation of Catholic Morals, p. 35)

“..there are many who honor Sacred Scripture, taking it as a norm of belief and a pattern of life, and who show a sincere zeal. They lovingly believe in God the Father Almighty and in Christ, the Son of God and Saviour. (Cf. Jn. 16:13) They are consecrated by baptism, in which they are united with Christ. They also recognize and accept other sacraments within their own Churches or ecclesiastical [Protestant] communities..

They also share with us in prayer and other spiritual benefits. Likewise we can say that in some real way they are joined with us in the Holy Spirit, for to them too He gives His gifts and graces whereby He is operative among them with His sanctifying power. Some indeed He has strengthened to the extent of the shedding of their blood...-LUMEN GENTIUM: 16.