4 Now, concerning workingmen's associations, even though their purpose is to obtain earthly advantages for their members, nonetheless those associations are to be most approved and considered as most useful for the genuine and permanent advantage of their members which are established chiefly on the foundation of the Catholic religion and openly follow the directives of the Church. We have repeated this declaration on several previous occasions in answer to question from various countries. Consequently, such so-called confessional Catholic associations must certainly be established and promoted in every way in Catholic regions as well as in all other districts where it can be presumed that they can sufficiently assist the various needs of their members. However, when there is a question about associations which directly or indirectly touch upon the sphere of religion and morality, it would not be permitted to foster and spread mixed organizations, that is, associations composed of Catholics and non-Catholics, in the areas just mentioned. Over and above other matters, in such organizations there are or certainly can be for our people serious dangers to the integrity of their faith and the due obedience to the commandments and precepts of the Catholic Church. Venerable Brethren, you yourselves have also openly called attention to this question in several of your answers which We have read.
Source: Singulari Quadam (Vatican.va)