146 Venerable Brethren and Beloved Sons, let us not permit the children of this world to appear wiser in their generation than we who by the Divine Goodness are the children of the light. We find them, indeed, selecting and training with the greatest shrewdness alert and resolute devotees who spread their errors ever wider day by day through all classes of men and in every part of the world. And whenever they undertake to attack the Church of Christ more violently, We see them put aside their internal quarrels, assembling in fully harmony in a single battle line with a completely united effort, and work to achieve their common purpose.
Source: Quadragesimo Anno (Vatican.va)