In addition , we can claim a threefold unity of the Evangelical churches. The first is canonical unity, by which - namely, with the canonical books of the Old and New Testaments - it agrees in everything with our doctrine that we profess, for no Papist has ever been able to convict us of any error in the articles of faith on the basis of Holy Scripture. In fact, the foremost Papist writers were forced to admit that they cannot overturn our confession at all on the basis of Holy Scripture.
The second is ecclesiastical unity, by which - namely, with the church writers whom they call "fathers" and especially with those who were closest to the times of the apostles - it agrees with our doctrine. For we are ready in every article in controversy to provide clear and manifest testimonies from the fathers in favor of our position.
The third is symbolical unity, a unity in which we embrace by common consent the doctrine contained in the Symbolical books of our churches... If anyone refuses to give his name to this, we do not recognize him as a brother in the matter of faith and confession. -- Blessed Johann Gerhard, On the Church, pp. 522,523.