4 It is so evident from the clear and frequent testimonies of Holy Writ that the true Church of Jesus Christ is one , that no Christian can dare to deny it. But in judging and determining the nature of this unity many have erred in various ways. Not the foundation of the Church alone, but its whole constitution, belongs to the class of things effected by Christ's free choice. For this reason the entire case must be judged by what was actually done. We must consequently investigate not how the Church may possibly be one, but how He, who founded it, willed that it should be one. But when we consider what was actually done we find that Jesus Christ did not, in point of fact, institute a Church to embrace several communities similar in nature, but in themselves distinct, and lacking those bonds which render the Church unique and indivisible after that manner in which in the symbol of our faith we profess: "I believe in one Church." "The Church in respect of its unity belongs to the category of things indivisible by nature, though heretics try to divide it into many parts...We say, therefore, that the Catholic Church is unique in its essence, in its doctrine, in its origin, and in its excellence...Furthermore, the eminence of the Church arises from its unity, as the principle of its constitution - a unity surpassing all else, and having nothing like unto it or equal to it" (S. Clemens Alexandrinus, Stronmatum lib. viii., c. 17). For this reason Christ, speaking of the mystical edifice, mentions only one Church, which he calls His own-"I will build my church;" any other Church except this one, since it has not been founded by Christ, cannot be the true Church. This becomes even more evident when the purpose of the Divine Founder is considered. For what did Christ, the Lord, ask? What did He wish in regard to the Church founded, or about to be founded? This: to transmit to it the same mission and the same mandate which He had received from the Father, that they should be perpetuated. This He clearly resolved to do: this He actually did. "As the Father bath sent me, I also send you" (John xx., 21). "Ad thou bast sent Me into the world I also have sent them into the world" (John xvii., 18). But the mission of Christ is to save that which had perished : that is to say, not some nations or peoples, but the whole human race, without distinction of time or place. "The Son of Man came that the world might be saved by Him" (John iii., 17). "For there is no other name under Heaven given to men whereby we must be saved" (Acts iv., 12). The Church, therefore, is bound to communicate without stint to all men, and to transmit through all ages, the salvation effected by Jesus Christ, and the blessings flowing there from. Wherefore, by the will of its Founder, it is necessary that this Church should be one in all lands and at all times. to justify the existence of more than one Church it would be necessary to go outside this world, and to create a new and unheard - of race of men. That the one Church should embrace all men everywhere and at all times was seen and foretold by Isaiah, when looking into the future he saw the appearance of a mountain conspicuous by its all surpassing altitude, which set forth the image of "The House of the Lord" - that is, of the Church, "And in the last days the mountain of the House of the Lord shall be prepared on the top of the mountains" ( Isa . ii., 2). But this mountain which towers over all other mountains is one ; and the House of the Lord to which all nations shall come to seek the rule of living is also one. "And all nations shall flow into it. And many people shall go, and say: Come, and let us go up to the mountain of the Lord, and to the House of the God of Jacob, and He will teach us His ways, and we will walk in His paths" ( Ibid ., ii., 2-3). Explaining this passage, Optatus of Milevis says: "It is written in the prophet Isaiah: 'from Sion the law shall go forth and the word of the Lord from Jerusalem.' For it is not on Mount Sion that Isaiah sees the valley, but on the holy mountain, that is, the Church, which has raised itself conspicuously throughout the entire Roman world under the whole heavens....The Church is, therefore, the spiritual Sion in which Christ has been constituted King by God the Father, and which exists throughout the entire earth, on which there is but one Catholic Church" ( De Schism. Donatist ., lib. iii., n. 2). And Augustine says: "What can be so manifest as a mountain, or so well known? There are, it is true, mountains which are unknown because they are situated in some remote part of the earth.....But this mountain is not unknown; for it has filled the whole face of the world, and about this it is said that it is prepared on the summit of the mountains" ( In Ep . Joan ., tract i., n. 13).
Source: Satis Cognitum (Vatican.va)