Apologeticae sive Theologiae Fundamentalis, Tomus II (Pars Altera): Demonstratio Catholica

In explicit words Christ affirms that the Church is unique. For speaking of the institution of the Church, He says: "Upon this rock I will build my Church."¹ He says that He will build not several churches, but one Church of His upon one rock. Elsewhere, discoursing of the Church already built, He says: "And other sheep I have, which are not of this fold [scil. from the people and synagogue of the Jews]: them also [the Gentiles] I must bring, and they shall hear my voice, and there shall be one fold and one shepherd."² Therefore the faithful from Jews and Gentiles are to be gathered into one and the same Church under one shepherd. Moreover, Christ preaches the one kingdom of God, embracing the whole world and consequently excluding every partition and plurality.

In deeds Christ instituted only one Church, in that He set one and the same authority over the multitude of the faithful. And so it is, as abundantly appears from what has been demonstrated. For He chose the one college of apostles to teach, sanctify, and direct all the faithful, and He strictly commanded all the faithful to be subject to this one authority. Nay more, over the college of apostles itself and over the whole flock of the faithful He set one Peter, that he might be the foundation, the key-bearer, and the shepherd of the whole Church.³ Therefore, by the intention of Christ, the Church excludes plurality.