In 1675 Philipp Spener wrote a preface to a book of sermons by his hero Johann Arndt. This preface became so popular it was soon published as a book in its own right by the name of Pia Desideria, Latin for “Pious Desires.” In it, he outlines what had gone wrong with the Lutheran church in his era and he makes some proposals for how it might be fixed. Here’s what he suggested in how to conduct ourselves in religious debates with unbelievers and heretics:

We owe it to the erring, first of all, to pray earnestly that the good God may enlighten them with the same light with which he blessed us, may lead them to the truth, may prepare their hearts for it or, having countered their dangerous errors, may reinforce what true knowledge of salvation in Christ they still have left in order that they may be saved as a brand plucked from the fire.

In the second place, we must give them a good example and take the greatest pains not to offend them in any way, for this would give them a bad impression of our true teaching and hence would make their conversion more difficult.

In the third place, if God has given us the gifts which are needful for it and we find the opportunity to hope to win the erring, we should be glad to do what we can to point out, with a modest but firm presentation of the truth we profess, how this is based on the simplicity of Christ’s teaching. At the same time we should indicate decently but forcefully how their errors conflict with the Word of God and what dangers they carry in their wake. All of this should be done in such a way that those with whom we deal can see for themselves that everything is done out of heartfelt love toward them, without carnal and unseemly feelings, and that if we ever indulge in excessive vehemence this occurs out of pure zeal for the glory of God. Especially should we beware of invectives and personal insinuations, which at once tear down all the good we have in mind to build.

To this should be added, in the fourth place, a practice of heartfelt love toward all unbelievers and heretics. While we should indicate to them that we take no pleasure in their unbelief or false belief or the practice and propagation of these, but rather are vigorously opposed to them, yet in other things which pertain to human life we should demonstrate that we consider these people to be our neighbors (as the Samaritan was represented by Christ in Luke 10:29-37 as the Jew’s neighbor), regard them as our brothers according to the right of common creation and the divine love that is extended to all (though not according to regeneration), and therefore are so disposed in our hearts toward them as the command to love all others as we love ourselves demands. To insult or wrong an unbeliever or heretic on account of his religion would be not only a carnal zeal but also a zeal that is calculated to hinder his conversion.

In the fifth place, if there is any prospect of a union of most of the confessions among Christians, the primary way of achieving it, and the one that God would bless most, would perhaps be this: that we do not stake everything on argumentation, for the present disposition of men’s minds, which are filled by as much fleshly as spiritual zeal, makes disputation fruitless. It is true that defense of the truth, and hence also argumentation, which is part of it, must continue in the church together with other things instituted to build it up. Nevertheless, I adhere to the splendidly demonstrated assertion of our sainted Arndt: “Purity of doctrine and of the Word of God is maintained not only by disputation and writing many books but also by true repentance and holiness of life.” Thus holiness of life itself contributes much to conversion, as Peter teaches (I Pet. 3:1-2).