Pope Pius XI
Quadragesimo Anno §62
Quadragesimo Anno: On Reconstruction of the Social Order
62 All these things which Our Predecessor has not only suggested but clearly and openly proclaimed, We emphasize with renewed insistence in our present Encyclical; and unless utmost efforts are made without delay to put them into effect, let no one persuade himself that public order, peace, and the tranquillity of human society can be effectively defended against agitators of revolution.
Source: Quadragesimo Anno (Vatican.va)