31 It is for you, therefore, venerable brethren, whom Divine Providence has constituted to be the pastors and leaders of the Christian people, to resist with all your strength this most fatal tendency of modern society to lull itself in a shameful indolence while war is being waged against religion, seeking a cowardly neutrality made up of weak schemes and compromises to the injury of divine and human rights, to the oblivion of Christ's clear sentence: "He that is not with me is against me" ( Matt . xii. 30). Not indeed that it is not well at times to waive our rights as far as may lawfully be done and as the good of souls requires. And certainly this defect can never be charged to you who are spurred on by the charity of Christ. But this is only a reasonable condescension, which can be made without the slightest detriment to duty, and which does not at all affect the eternal principles of truth and justice.
Source: Communium Rerum (Vatican.va)