Yet despite these difficulties, which long postponed its convocation and repeatedly interrupted its labors, the Council of Trent [Footnote: Trent was selected largely by reason of its geographical location, being situated on the boundary between the German-speaking and Italian- speaking peoples.] consummated a great reform in the Church and contributed materially to the preservation of the Catholic faith. The Protestants, whom the pope invited to participate, absented themselves; yet such was the number and renown of the Catholic bishops who responded to the summons that the Council of Trent easily ranked with the eighteen œcumenical councils which had preceded it. [Footnote: Its decrees were signed at its close (1563) by 4 cardinal legates, 2 cardinals, 3 patriarchs, 25 archbishops, 167 bishops, 7 abbots, 7 generals of orders, and 19 proxies for 33 absent prelates.] The work of the council was twofold--dogmatic and reformatory. Dogmatically, the fathers at Trent offered no compromise to the Protestants. They confirmed with inexorable frankness the main points in Catholic theology which had been worked out in the thirteenth century by Thomas Aquinas and which before the appearance of Protestantism had been received everywhere in central and western Europe. They declared that the tradition of the Church as well as the Bible was to be taken as the basis of the Christian religion, and that the interpretation of the Holy Scripture belonged only to the Church. The Protestant teachings about grace and justification by faith were condemned, and the seven sacraments were pronounced indispensable. The miraculous and sacrificial character of the Lord's Supper (Mass) was reaffirmed. Belief in the invocation of saints, in the veneration of images and of relics, in purgatory and indulgences was explicitly stated, but precautions were taken to clear some of the doctrines of the pernicious practices which at times had been connected with them. The spiritual authority of the Roman See was confirmed over all Catholicism: the pope was recognized as supreme interpreter of the canons and incontestable chief of bishops. [Sidenote: Reformatory Canons of the Council of Trent ] A volume of disciplinary statutes constituted the second achievement of the Tridentine Council. The sale of church offices was condemned. Bishops and other prelates were to reside in their respective dioceses, abandon worldly pursuits, and give themselves entirely to spiritual labors. Seminaries were to be established for the proper education and training of priests. While Latin was retained as the official and liturgical language, frequent sermons were to be preached in the vernacular. Indulgences were not to be issued for money, and no charge should be made for conferring the sacraments. [Sidenote: Index and inquisition ]