"The apostles and their successors are God's vicars in governing the Church which is built on faith and the sacraments of faith. Wherefore, just as they may not institute another Church, so neither may they deliver another faith, nor institute other sacraments."

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Just as in a physical body the operation of one member contributes to the good of the whole body, so it is in a spiritual body such as the Church. And since all the faithful are one body, the good of one member is communicated to another; everyone members, as the Apostle says, of one another [Eph 4:25]. For that reason, among the points of faith handed down by the Apostles, is that there is a community of goods in the Church, and this is expressed in the words Communion of Saints.

— Thomas Aquinas