The priest not only forgave sin; he was also empowered to perform the stupendous miracle of the Mass. The early Christians had celebrated the Lord's Supper or Holy Eucharist in various ways and entertained various conceptions of its nature and significance. Gradually the idea came to be universally accepted that by the consecration of the bread and the wine the whole substance of the bread was converted into the substance of the body of Christ, and the whole substance of the wine into his blood. This change was termed _transubstantiation_. The Church believed, further, that in this sacrament Christ was offered up anew, as he had been on the cross, as a sacrifice to God. This sacrifice might be performed for the sins of the absent as well as of the present, and for the dead as well as for the living. Moreover, Christ was to be worshiped under the form of the bread, or _host_ (Latin, _hostia_, sacrifice), with the highest form of adoration. The host was to be borne about in solemn procession when God was to be especially propitiated, as in the case of a famine or plague. [Sidenote: Consequences of conceiving the Mass as a sacrifice.] This conception of the Mass as a sacrifice had some important practical consequences. It became the most exalted of the functions of the priest and the very center of the Church's services. Besides the public masses for the people, private ones were constantly celebrated for the benefit of individuals, especially of the dead. Foundations were created, the income of which went to support priests for the single purpose of saying daily masses for the repose of the soul of the donor or those of the members of his family. It was also a common practice to bestow gifts upon churches and monasteries on condition that annual or more frequent masses should be said for the giver. [Sidenote: The dominant position of the clergy and the sources of their power.] [Sidenote: Excommunication and interdict.] 83. The sublime prerogatives of the Church, together with its unrivaled organization and vast wealth, combined to make its officers, the clergy, the most powerful social class of the Middle Ages. They held the keys of heaven and without their aid no one could hope to enter in. By excommunication they could not only cast an offender out of the Church, but also forbid his fellow-men to associate with him, since he was accursed and consigned to Satan. By means of the _interdict_ they could suspend the consolations of religion in a whole city or country by closing the church doors and prohibiting all public services.[144] [Sidenote: Their monopoly of the advantages of education.]