His strongest attack on the existing ecclesiastical system was made in a sermon on tithes, which, to the distress of the Provost of the Minster, he declared to be merely voluntary offerings. (He had been reading Hus' book _On the Church_.) He must have carried most of the Chapter with him in his schemes for improvement, for in June 1520 the Breviary used in the Minster was revised by Zwingli and stripped of some blemishes. In the following year (March 1521), some of the Zurichers who were known to be among Zwingli's warmest admirers, the printer Froschauer among them, asserted their convictions by eating flesh meat publicly in Lent. The affair made a great sensation, and the Reformers were brought before the Council of the city. They justified themselves by declaring that they had only followed the teaching of Zwingli, who had shown them that nothing was binding on the consciences of Christians which was not commanded in the Scriptures. Zwingli at once undertook their defence, and published his sermon, _Selection or Liberty concerning Foods; an offence and scandal; whether there is any Authority for forbidding Meat at certain times_ (April 16th, 1522). He declared that in such matters the responsibility rests with the individual, who may use his freedom provided he avoids a public scandal. The matter was felt to be serious, and the Council, after full debate, passed an ordinance which was meant to be a compromise. It was to the effect that although the New Testament makes no rule on the subject, fasting in Lent is a very ancient custom, and must not be set aside until dealt with by authority, and that the priests of the three parishes of Zurich were to dissuade the people from all violation of the ordinance. The Bishop of Constance thereupon interfered, and sent a Commission, consisting of his suffragan and two others, to investigate and report. They met the Small Council, and in a long address insisted that the Church had authority in such matters, and that the usages it commanded must be obeyed. Zwingli appeared before the Great Council, and, in spite of the efforts of the Commission to keep him silent, argued in defence of liberty of conscience. In the end the Council resolved to abide by its compromise, but asked the Bishop of Constance to hold a Synod of his clergy and come to a resolution upon the matter which would be in accordance with the law of Christ. This resolution of the Council really set aside the episcopal authority, and was a revolt against the Roman Church.