Notre charge apostolique
"Here we have, founded by Catholics, an inter-denominational association that is to work for the reform of civilization, an undertaking which is above all religious in character; for there is no true civilization without a moral civilization, and no true moral civilization without the true religion: it is a proven truth, a historical fact. The new Sillonists cannot pretend that they are merely working on “the ground of practical realities” where differences of belief do not matter. Their leader is so conscious of the influence which the convictions of the mind have upon the result of the action, that he invites them, whatever religion they may belong to, “to provide on the ground of practical realities, the proof of the excellence of their personal convictions.” And with good reason: indeed, all practical results reflect the nature of one’s religious convictions, just as the limbs of a man down to his finger-tips, owe their very shape to the principle of life that dwells in his body.This being said, what must be thought of the promiscuity in which young Catholics will be caught up with heterodox and unbelieving folk in a work of this nature? Is it not a thousand-fold more dangerous for them than a neutral association?"
(Pope St. Pius X, Apostolic Letter Notre Charge Apostolique; underlining added.)