In a special pamphlet, ' Ueber die Treue welche man den Haretikern schuldet,' Fr. Martin Becanus, S.J. lays down the general rule: ' If you have concluded a treaty or an alliance with heretics, you must thoroughly and honourably fulfil that which you have promised, just as much as you would in the case of Catholics.' For ' we must never tell lies, never violate our neighbours' rights, never commit an act of injustice, never be guilty of perjury. In very deed, if you once admit that all such wrong-doing is allowable on the ground that you are dealing with a heretic, it follows that you also have the right to kill, rob, and hate heretics; but this would be contrary to reason and to the law of God.' Even towards unbelievers and worshippers of idols, according to the testimony of Holy Writ, faith must be observed —how much more, then, to the Protestant sects ?
He then brings forward some specially important cases to show how we are bound to keep faith even with outlaws, excommunicated persons, and heretics, in marriage, in war, and in case of having granted a safeconduct. Charles V., for instance, did all honour to the name of a Catholic emperor when he refused at Worms to violate Martin Luther's safe-conduct. The most important section of the treatise bears the heading: 'Ob man den Haretikern Treue halten miisse, wenn es sich urn die Freiheit der Religion handelt' (' Whether it is necessary to keep faith with heretics, when it is a question of freedom of religion'). Becanus begins by reminding his readers that Christ's ideal, according to the testimony of the Gospel, was that men should have but one faith, one Church, and one supreme shepherd. A variety of religions in a State was dangerous, and disturbed the peace of civil life, as is seen from the history of the Donatists, the Iconoclasts, the Albigenses, the Hussites, and the Calvinists in England, Belgium, France, and Poland. No Catholic prince, therefore, ought of his own accord to introduce religious freedom.
The greatest emperors of Christian antiquity, Fathers of the Church, such as Ambrosius, Chrysostom, Augustine, had striven with the utmost zeal to preserve to the Catholic Church alone the right of public worship of God. ' If, however, the Catholic ruling authorities in any given place are unable to prevent the existence of other modes of belief and worship side by side with the Catholic faith, without occasioning still worse evil to the community, they must then be allowed to tolerate the unorthodox religions.'This was the emphatic teaching of Thomas of Aquinas, and in the same sense, says Becanus, spoke the scholars of the Jesuit Order, Maldonat, Gregory of Valentia, and Molina. If, then, he says, in concluding his typical instances, a Catholic authority seals a contract with heretics with reference to toleration of this sort, ' there is no question whatever but that the contract must be adhered to; for the obligation of faith and loyalty arises out of every legitimate, honest compact. At the same time, however, it is permissible, and in accordance with the moral law, that freedom of religion be tolerated in order to avoid greater evil, and a Catholic prince has full right to make such toleration the subject-matter of a treaty; and, if he does so, he is bound in honour to keep his word.' In 1593 Peter Stevart, professor of theology at Ingolstadt, entreated the Emperor, princes, and Estates that ' for God's sake and for the establishment of truth they would plainly state whether they had ever received from the Society of Jesus any such instructions and counsels for the extermination of all the Evangelicals and Protestants.' ' For if your Imperial Majesty and your princely graces do really declare that the Jesuits are contemplating sanguinary onslaughts of this kind, our German nation will then come forward and call on your Imperial Majesty and the princes for vengeance against these insurrectionary people, and insist that they shall be at once sentenced to death.'
Link (here) to the fascinating book entitled, History of the German People at the end of the Middle Ages