It might help to understand that (as Vatican II taught) the Church is, first, a mystery. Don’t think of it first, or necessarily primarily, as the visible hierarchical, clearly defined time-bound institution (like the Rotary club).
Unless I’m wrong, the idea that there is “no salvation outside the Church” was attributed to St. Cyprian in the mid 3rd century (so it’s not quite 2000 years old)–and if there’s an earlier reference anyone knows of please share of course. At the time, of course, “the Church” meant ALL CHRISTIANS. This was before any schisms etc. that unfortunately divided the Body of Christ.
The Catholic Church IS the Mystical Body of Christ:
Pope Pius XII, Mystici Corporis Christi (On the Mystical Body of Christ), June 29, 1943:
“12… As He hung upon the Cross, Christ Jesus not only appeased the justice of the Eternal Father which had been violated, but He also won for us, His brethren, an ineffable flow of graces. It was possible for Him of Himself to impart these graces to mankind directly; but He willed to do so only through a visible Church made up of men, so that through her all might cooperate with Him in dispensing the graces of Redemption… 13. If we would define and describe this true Church of Jesus Christ—which is the One, Holy, Catholic, Apostolic Roman Church[12]—we shall find nothing more noble, more sublime, or more divine than the expression “the Mystical Body of Jesus Christ”—an expression which springs from and is, as it were, the fair flowering of the repeated teaching of the Sacred Scriptures and the holy Fathers. 14. That the Church is a body is frequently asserted in the Sacred Scriptures. “Christ,” says the Apostle, “is the Head of the Body of the Church.”[13] If the Church is a body, it must be an unbroken unity, according to those words of Paul: “Though many we are one body in Christ.”[14] But it is not enough that the body of the Church should be an unbroken unity; it must also be something definite and perceptible to the senses as Our predecessor of happy memory, Leo XIII, in his Encyclical Satis Cognitum asserts: “the Church is visible because she is a body.”[15] Hence they err in a matter of divine truth, who imagine the Church to be invisible, intangible, a something merely “pneumatological” as they say, by which many Christian communities, though they differ from each other in their profession of faith, are united by an invisible bond.”
ewtn.com/library/ENCYC/P12MYSTI.HTM
Actually the earliest statements are taken from Scripture and then we have early Church fathers as early as St. Ignatius of Antioch saying the following:
“Be not deceived, my brethren: If anyone follows a maker of schism
, he does not inherit the kingdom of God; if anyone walks in strange doctrine
, he has no part in the passion [of Christ]. Take care, then, to use one Eucharist, so that whatever you do, you do according to God: For there is one flesh of our Lord Jesus Christ, and one cup in the union of his blood; one altar, as there is one bishop, with the presbytery and my fellow servants, the deacons” (*Letter to the Philadelphians *3:3–4:1 [A.D. 110]).
And St. Irenaeus: “In the Church God has placed apostles, prophets, teachers, and every other working of the Spirit, of whom none of those are sharers who do not conform to the Church, but who defraud themselves of life by an evil mind and even worse way of acting. Where the Church is, there is the Spirit of God; where the Spirit of God is, there is the Church and all grace” (
Against Heresies 3:24:1 [A.D. 189]).
Regarding schisms, they were present from the beginning of the Church, and the Church was not merely understood as just people, but it was the people in union with the bishops, as the quote from Ignatius should help clarify. The Church is also described in Scripture as “the pillar and foundation of truth” (1 Tim 3:15). So even Scripture itself referrs to this universal body.
Matthew 18:17-18: 7 And if he will not hear them: tell the Church. And if he will not hear the Church, let him be to thee as the heathen and publican. 18 Amen I say to you, whatsoever you shall bind upon earth, shall be bound also in heaven: and whatsoever you shall loose upon earth, shall be loosed also in heaven.
The Church has always understood the underlined statement as referring to excommunication as attested to by the fathers and subsequent popes.
Spoken by Christ to his apostles and by extension to their successors: “He that hears you hears me: and he that despises you despises me: and he that despises me despises him that sent me” (Luke 10:16).