The mass was the very centre of all Church worship. Pope Gregory I. established its mediæval form. The celebration of the mass was the bloodless sacrifice of Christ to God for the world's sins, a reconciliation of heaven and earth, of benefit to the living and to the pious dead. It is no wonder then that the mass was celebrated several times daily with the greatest ritualistic pomp and display. Masses for the dead, too, became popular as the doctrine of purgatory developed[370:2] and were usually celebrated as solitary masses. Lullus even ordered masses and fasts in order to obtain good weather.[370:3] The dogma of transubstantiation while generally held had not yet become Church law. Church worship throughout western Europe was conducted in Latin, and consequently was little understood by the masses of the laity. Although preaching was not a necessary part of the regular Church service, still it was not an unusual feature. Pope Gregory I. frequently preached with great earnestness, although his successors did not follow his example. Bishops were required to preach, but their negligence was proverbial.[371:1] The priests were commanded to explain to their people the Apostles' Creed, the Lord's Prayer, and the nature of the sacraments. The models recommended were the homilies,[371:2] and the sermons of Gregory I.[371:3] The vernacular was used of course in all preaching and cathedral instruction. The Church hymns of this period reflect the Christian life and worship. In the Latin Church the hymns are divided into three periods: the patristic epoch to Gregory I. (d. 604); the mediæval epoch to Damiani (d. 1073); and the classical epoch to 1300. These Latin hymns possess much fervour and some genius, and have a very pronounced character. Most of them were inspired by the Blessed Virgin and next in favour came the saints. There were many beautiful products like _Te Deum Laudamus_.[371:4] In the early churches no organ was used.[371:5] Pope Vitalian (657-672) probably first employed one, while Pepin and Charles the Great both received presents of this instrument from the East. After the eighth century it was generally used during the Middle Ages.[372:1] Church bells gradually came into use after the time of Constantine and were very numerous during this period.[372:2]