7 Furthermore, if Catholics are to be permitted to join the trade unions, these associations must avoid everything that is not in accord, either in principle or practice, with the teachings and commandments of the Church or the proper ecclesiastical authorities. Similarly, everything is to be avoided in their literature or public utterances or actions which in the above view would incur censure. The Bishops, therefore, should consider it their sacred duty to observe carefully the conduct of all these associations and to watch diligently that the Catholic members do not suffer any harm as a result of their participation. The Catholic members themselves, however, should never permit the unions, whether for the sake of material interests of their members or the union cause as such, to proclaim or support teachings or to engage in activities which would conflict in any way with the directives proclaimed by the supreme teaching authority of the Church, especially those mentioned above. Therefore, as often as problems arise concerning matters of justice or charity, the Bishops should take the greatest care to see that the faithful do not overlook Catholic moral teaching and do not depart from it even a finger's breadth.
Source: Singulari Quadam (Vatican.va)