5 We have never ceased, Venerable Brethren, to represent to the responsible rulers of your country's destiny, the consequences which would inevitably follow the protection and even the favor, extended to such a policy. We have done everything in Our power to defend the sacred pledge of the given word of honor against theories and practices, which it officially endorsed, would wreck every faith in treaties and make every signature worthless. Should the day ever come to place before the world the account of Our efforts, every honest mind will see on which side are to be found the promoters of peace, and on which side its disturbers. Whoever had left in his soul an atom of love for truth, and in his heart a shadow of a sense of justice, must admit that, in the course of these anxious and trying years following upon the conclusion of the concordat, every one of Our words, every one of Our acts, has been inspired by the binding law of treaties. At the same time, anyone must acknowledge, not without surprise and reprobation, how the other contracting party emasculated the terms of the treaty, distorted their meaning, and eventually considered its more or less official violation as a normal policy. The moderation We showed in spite of all this was not inspired by motives of worldly interest, still less by unwarranted weakness, but merely by Our anxiety not to draw out the wheat with the cockle; not to pronounce open judgment, before the public was ready to see its force; not to impeach other people's honesty, before the evidence of events should have torn the mask off the systematic hostility leveled at the Church. Even now that a campaign against the confessional schools, which are guaranteed by the concordat, and the destruction of free election, where Catholics have a right to their children's Catholic education, afford evidence, in a matter so essential to the life of the Church, of the extreme gravity of the situation and the anxiety of every Christian conscience; even now Our responsibility for Christian souls induces Us not to overlook the last possibilities, however slight, of a return to fidelity to treaties, and to any arrangement that may be acceptable to the episcopate. We shall continue without failing, to stand before the rulers of your people as the defender of violated rights, and in obedience to Our Conscience and Our pastoral mission, whether We be successful or not, to oppose the policy which seeks, by open or secret means, to strangle rights guaranteed by a treaty.
Source: Mit Brennender Sorge (Vatican.va)