Section — Wouters, Manual of Moral Theology
Section III. On Dangers of Faith
The principal dangers of faith are:
I. Reading of impious writings; concerning this, see the treatise on the precepts of the Church, and on Penance, where occasions of sin are discussed.
II. Marriages with non-Catholics, concerning which see the treatise on Marriage.
III. Attendance at non-Catholic schools; concerning this, see the treatise on the fourth precept of the Decalogue.
IV. Familiarity with non-Catholics; concerning this, let these words of the Apostle be noted: "Avoid a heretical man...";1 likewise: "Their speech spreads like a cancer";2 likewise: "Avoid them. For such persons do not serve Christ our Lord, but their own belly; and by sweet speeches and blessings they seduce the hearts of the innocent."3 To this also should be referred service among non-Catholics, concerning which see the treatise on states of life. Besides, the ecclesiastical laws which regarded association with Jews, and which in most regions had almost been abrogated by custom, are omitted in the new Code, and therefore, insofar as they were merely ecclesiastical, have been removed. Hence attention must be paid only to natural law.
1 Titus 3:10.
2 2 Tim. 2:17.
V. Communication with non-Catholics in sacred and mixed matters. Active communication (n. 500) is prohibited not only by reason of the danger of losing the faith, but also either as cooperation in evil, as if someone asks for a sacrament from a non-Catholic, or as profession of a false religion, as if someone participates in a non-Catholic rite, for example by singing psalms with non-Catholics, receiving a heretical supper, etc. (n. 500). It is licit to pray orthodox prayers privately with them.
Concerning passive communication, see what was said in n. 500.
VI. Associations to be reproved. "The faithful are worthy of praise if they give their names to associations erected or at least commended by the Church; but let them beware of secret, condemned, seditious, suspected associations, or associations which strive to withdraw themselves from the legitimate vigilance of the Church" (can. 684).
Corollary I. It is illicit to give one's name to anti-Catholic associations, whether religious or social, of which sort are, for example:
Masonic societies;1 Carbonari;2 Fenians;3 Anarchists;4 Communists, Nihilists, Internationalists, and Socialists;5 theosophical societies;6 biblical societies;7 clerico-liberal societies;8 a society erected for procuring the unity of Christianity;9 workers' societies which are called Odd Fellows, Sons of Temperance, Knights of Pythias;10 the Independent Order of Good Templars (Guttempler-Orden);11 and societies which promote the cremation of human corpses.12
Corollary II. Per se it is also illicit to give one's name to social associations, whether indeed Christian but not specifically Catholic, or neutral. The reason is that social questions too must be solved with dependence on the Catholic Church, and not, as happens in the aforesaid associations, independently of her (n. 117). There is added that dangers to faith and morals are not so rarely attached to societies of this sort.
Corollary III. By reason of dangers, other associations or parties also can be prohibited by ecclesiastical authority, as, for example, Action Française.13
Corollary IV. As regards the gravity of the sin committed by one who gives his name to an association to be reproved, it must be said as follows:
One who, as is commonly done by those of whom we are speaking, internally admits some proposition reproved by the Church sins gravely.
One who internally does not admit any doctrine definitively proscribed by the Church nevertheless sins gravely:
a) If he gives his name to an anti-Catholic association, whether religious or social; both because at least externally he professes errors rejected by the Church, and because there are added cooperation, scandal, and often danger to faith, arising from the fact that he has given his name to a party which professes evil principles, pursues evil ends, and uses evil means.
b) If he gives his name to an association or party named and truly prohibited by ecclesiastical authority, because a prohibition of this sort is judged to have been made under grave obligation.
c) It is not clear that the same must per se be said of one who gives his name to a social association, whether Christian but not specifically Catholic, or neutral, but nevertheless not positively prohibited. In the case, a grave cause can excuse, unless scandal to be avoided because of the circumstances stands in the way.
1 Can. 2335.
2 Cf. Constitution of Pius VII Ecclesiam, September 25, 1821, and Leo XII, Quo graviora, March 13, 1825; cited canon.
3 Cf. Holy Office, January 12, 1870; cited canon.
4 Cited canon.
5 Encyclical of Leo XIII Quod apostolici muneris, December 28, 1878; Encyclical of Pius XI Quadragesimo anno, May 15, 1931.
6 Cf. Holy Office, July 18, 1919 (Acta Apostolicae Sedis 1919, p. 317).
7 Constitution of Pius IX Qui pluribus, 1846.
8 Syllabus, § 178; Encyclical of Pius IX Quanta cura, December 8, 1864; Encyclical of Leo XIII Libertas, June 20, 1888.
9 Cf. Holy Office, September 16, 1864, November 8, 1865 (Collectanea de Propaganda Fide, nn. 1262, 1276).
10 Holy Office, August 20, 1894.
11 Holy Office, August 9, 1893 (Collectanea, n. 1845).
12 Holy Office, May 19, 1886 (Acta Sanctae Sedis, t. 19, p. 46).
13 Cf. Acta Apostolicae Sedis 1926, pp. 382, 517, 529; 1927, pp. 5, 157.