6 Not a few of you, Venerable Brethren, have asked Us whether it is permissible to tolerate the so-called Christian Trade Unions that now exist in your dioceses, since, on the one hand, they have a considerably larger number of members than the purely Catholic associations and, on the other hand, if permission were denied serious disadvantages would result. In view of the particular circumstances of Catholic affairs in Germany, We believe that We should grant this petition. Furthermore, We declare that such mixed associations as now exist within your dioceses can be tolerated and Catholics may be permitted to join them, as long as such toleration does not cease to be appropriate or permissible by reason of new and changed conditions. Necessary precautions, however, must be adopted in order to avoid the dangers which, as has already been mentioned, follow upon such associations. The following are the most important of these precautions: In the first place, provision should be made that Catholic workers who are members of the trade unions must also belong to those Catholic associations which are known as Arbeitervereine . In the event that they must make some sacrifice for this cause, even in a monetary way, We are convinced that they will readily do so for the sake of safeguarding the integrity of their Faith. As has been happily demonstrated, the Catholic workingmen's associations, aided by the clergy and by its leadership and alert direction, are able to achieve very much toward preserving the truths of religion and the purity of morals among their members, and nourish the religious spirit through frequent practices of piety. Therefore, the leaders of such associations, clearly recognizing the needs of the age, are undoubtedly prepared to instruct the workers about their duties in justice and charity, especially regarding all those commandments and precepts in which an accurate knowledge is needed or useful in order to enable them to take an active part in their trade unions according to the principles of Catholic doctrine.
Source: Singulari Quadam (Vatican.va)