It is practically important in that it affords a clue to anyone desirous of ascertaining to which of the competing religious bodies he should belong. It was usually to this article on the Church that those who afterwards returned to Catholicism appealed in vindication of their step. It was also the practice of Catholic writers, in their controversies with Luther, to appeal to the doctrine of the one Church which has never erred in dogma in order to convict him more speedily of the guilt of his separation. All of them started from the old definition, according to which the Church is the visible commonwealth of the faithful, founded by Christ on Peter, the Rock, which confesses the same Christian belief and unites in the same Sacraments under the guidance of its lawful pastors, in particular of the successors of St. Peter. Luther himself was fully aware of the supreme importance of this doctrine; he frequently enough brings his opponents on the scene “crying Church, Church!”[1081] Among the Papists, he says, they do nothing but shriek Church, Church, Church, and this is the chief obstacle to reunion.[1082] “Hence there is indeed need that we should see what the Holy Christian Church is. If it is the clergy and their mob, then the devil has won and we two, God and His Word, are the losers.”[1083] “The Pope quotes this text [John xiv. 17: ‘The spirit of truth shall remain with you’] strongly and impressively.… They have become so certain of their cause that they take their stand on it as on a wall of iron.… This we ourselves must believe and say, viz. that the Holy Ghost is with the Church which is certainly on earth and will remain.”[1084] But was Luther’s Church a visible or an invisible one? _Invisibility of Luther’s Church_ Bearing in mind the religious compulsion practised by Luther, the question would seem already answered. His practice involved the existence of an outward ecclesiastical authority with outward rules, a congregation to which it was impossible to belong without submitting to the doctrine of a visible head or corporation. Of the visible nature of this Church there can be no question. It is with this tangible authority that he confronts the Anabaptists, for instance when he says: “The presumption of these fanatics is unbearable, for they altogether repudiate the authority of the Church and will have it all their own way.”[1085] The best-grounded maxims of the best teachers are despised by them, so he complains, and they only esteem the opinions they themselves have rummaged for in Scripture! “Yet great heed should be paid to the Church.”[1086] Nevertheless, according to Luther’s own views which had not changed much since 1519, the Church is in reality invisible.