His course of conduct was eminently prudent. He invariably took pains to educate the people up to further changes by explaining them carefully in sermons, and by publishing and circulating these discourses. He considered that it was his duty to teach, but that it belonged to the civic authorities to make the changes; and he himself made none until they were authorised. He had very strong views against the use of images in churches, and had preached vigorously against their presence. Some of his more ardent hearers began to deface the statues and pictures. The Great Council accordingly took the whole question into consideration, and decided that a second Public Disputation should be held, at which the matter might be publicly discussed. This discussion (October 1523) lasted for two days. More than eight hundred persons were present, of whom three hundred and fifty were clergy. On the first day, Zwingli set forth his views on the presence of images in churches, and wished their use forbidden. The Council decided that the statues and pictures should be removed from the churches, but without disturbance; the rioters were to be pardoned, but their leader was to be banished from the city for two years. The second day's subject of conference was the Mass. Zwingli pled that the Mass was not a sacrifice, but a memorial of the death of our Lord, and urged that the abuses surrounding the simple Christian rite should be swept away. The presence of Anabaptists at this conference, and their expressions in debate, warned the magistrates that they must proceed cautiously, and they contented themselves with appointing a commission of eight--two from the Council and six clergymen--to inquire and report. Meanwhile the clergy were to be informed how to act, and the letter of instruction was to be written by Zwingli. The authorities also deputed preachers to go to the outlying parts of the canton and explain the whole matter carefully to the people. The letter which Zwingli addressed to the clergy of Zurich canton is a brief statement of Reformation principles. It is sometimes called the _Instruction_. Zwingli entitles it, _A brief Christian Introduction which the Honourable Council of the city of Zurich has sent to the pastors and preachers living in its cities, lands, and wherever its authority extends, so that they may henceforth in unison announce and preach the gospel._[16] It describes sin, the law, God's way of salvation, and then goes on to speak of images. Zwingli's argument is that the presence of statues and pictures in churches has led to idolatry, and that they ought to be removed. The concluding section discusses the Mass. Here the author states very briefly what he elaborated afterwards, that the main thought in the Eucharist is not the repetition of the sacrifice of Christ, but its faithful remembrance, and that the Romish doctrine and ceremony of the Mass has been so corrupted to superstitious uses that it ought to be thoroughly reformed.