At Wittenberg Luther felt himself completely at home. He was proud of the distinctions conferred upon him by his brethren, and of the influence accorded to him by his companions in the university. Great as were his industry and his powers of application, yet they were put to the most severe tests to enable him to complete the programme he had set himself to accomplish. His lectures at the university, his sermons preached in the Augustinian church, his visitations of the houses of his order in the district over which he was vicar, his correspondence, partly routine and partly entailed by his close relations with some of the leading men in Germany, occupied all his time even to the exclusion of the spiritual exercises enjoined by his rule. Very frequently he neglected to celebrate Mass or even to read the divine office, and then alarmed by his negligence and guilt he had recourse to extraordinary forms of penance. Fits of laxity were followed by fits of scrupulousness until at last he was driven at times almost to despair. It was then that he called to mind the consoling advice given to him by his superior that he should put his trust in the merits of Christ, and the teaching of St. Augustine on the frailty of human nature unless it was aided and supported by divine Grace. He began to develop the idea that justification could not be acquired by good works, that concupiscence could not be overcome, and that consequently man could be justified only by the imputation of the merits of Christ. Years before, views such as these had been passing through his mind, as may be seen in his sermons against the Augustinians of the strict observance, but they found adequate expression only in his commentaries on the Epistles of St. Paul to the Romans and to the Galatians (1515-6). Still, as yet, he held strongly to the principle of authority in matters of religion, and inveighed against heretics who would dare to set aside the authority of the Pope in order to follow their own judgment. In reality, however, his own teaching on merit and justification was no longer in harmony with Catholic doctrine, and only a slight occasion was required to bring him into open and definite conflict with the authorities of the Church.

This occasion was provided by the preaching in Germany of an Indulgence proclaimed by Leo X. (1513-21). The building of St. Peter's had been begun by Julius II. and was continued by his successor Leo X., the son of Lorenzo de' Medici, and the great patron of the Humanist movement. In order to provide funds to enable him to continue this gigantic undertaking Leo X. proclaimed an Indulgence. In addition to Confession and Holy Communion it was ordered that those of the faithful who wished to share in the spiritual favours granted by the Pope should contribute according to their means for the completion of St. Peter's, or that they should pray for the success of the work in case poverty did not permit them to give alms. The publication of the Indulgence in a great part of Germany was entrusted to Albrecht of Brandenberg, who had been elected Archbishop of Mainz though he was already Archbishop of Magdeburg and Administrator of Halberstadt. The fees to be paid by an archbishop appointed to Mainz were exceptionally high not to speak of the large sum required for the extraordinary favour of being allowed to hold two archbishoprics. As a means of enabling Albrecht to raise the required amount, it was proposed by an official of the Datary that he should be allowed to retain half of the contributions given on the occasion of the publication of the Indulgence in the provinces of Mainz and Magdeburg, and in the lands of the House of Brandenburg.

To publish the Indulgence in the above-mentioned territories Albrecht appointed the Dominican John Tetzel,[8] who had acquired already considerable renown as a preacher. Tetzel was a man of solid education and of good moral standing, whose reputation as a successful popular preacher stood high in Germany at this period. Many grave abuses have been alleged against him by his enemies concerning his manner of carrying out the office entrusted to him by the archbishop, and in regard to his own private life serious crimes have been laid to his charge; but as a matter of history it is now admitted that Tetzel was a much maligned man, that his own conduct can bear the fullest scrutiny, and that in his preaching the worst that can be said against him is that he put forward as certainties, especially in regard to gaining indulgences for the souls of the faithful departed, what were merely the opinions of certain schools of theologians. Nor is it true to say that as the result of his activity vast sums of money made their way into the papal treasury. The accounts of the monies received during the greater portion of the time are now available, and it can be seen that when all expenses were paid comparatively little remained for either the Archbishop of Mainz or the building fund of St. Peter's.[9]