39 Every form of social action involves some doctrine; and the Christian rejects that which is based on a materialistic and atheistic philosophy, namely one which shows no respect for a religious outlook on life, for freedom or human dignity. So long as these higher values are preserved intact, however, the existence of a variety of professional organizations and trade unions is permissible. Variety may even help to preserve freedom and create friendly rivalry. We gladly commend those people who unselfishly serve their brothers by working in such organizations.
Source: Populorum Progressio (Vatican.va)