As the mystical body of Christ is the only one that wholly consists of believers, that is the one true church, while competition remains for those who will claim to be the one true visible church.
Like faith without works, the universal church, into which a believer is added at the moment of conversion, (1Coir. 12:13) even if in the desert (Acts 8:36-38) or before baptism (Acts 10:43-47; 15:7-9) always has a visible expression. And it is the Biblical gospel of grace - which can result in immediate conversion by convicted sinners, destitute of moral fitness and damned due to the lack of it, placing faith in the mercy of the Biblical God in the Biblical Christ for salvation on His blood expense and righteousness - that is the core truth of the church. This does relate to more than just the facts of the the gospel, as conviction of sin, righteousness and judgment presumes at least light of conscience, and trusting Christ to save infers Deity, yet as the Bible shows, the truth that results in the body of Christ having its members can be apprehended by a prepared soul quite readily.
And as by faith in the gospel of basic truths is how the church gains its members, then it militates against one particular group claiming to be the only true visible church. It is those who distort that and require membership in their particular church for salvation, as they are exclusive possessors of salvific truth, and requiring implicit trust in men, who are marked as “cults.”
An Eastern Orthodox bishop has expressed this doctrine as follows:[2]
“Extra Ecclesiam nulla salus. All the categorical strength and point of this aphorism lies in its tautology. Outside the Church there is no salvation, because salvation is the Church” (G. Florovsky, “Sobornost: the Catholicity of the Church”, in The Church of God, p. 53).
Does it therefore follow that anyone who is not visibly within the Church is necessarily damned? Of course not; still less does it follow that everyone who is visibly within the Church is necessarily saved.
As Augustine wisely remarked: “How many sheep there are without, how many wolves within!” (Homilies on John, 45, 12) While there is no division between a “visible” and an “invisible Church”, yet there may be members of the Church who are not visibly such, but whose membership is known to God alone. If anyone is saved, he must in some sense be a member of the Church; in what sense, we cannot always say. —Kallistos Ware
Protestants cannot say no one is saved in the RCC, nor can it claim it alone is the body of Christ, and it does nor, but the contention is when either is inferred, and which best represents the Bilical church, and upon what basis is that determined.
Daniel — the Catechism states of “separated Churches and Communities as such”, that they “have been by no means deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Church.”
II. The visible Church, which is also catholic or universal under the Gospel (not confined to one nation, as before under the law), consists of all those throughout the world that profess the true religion;[2] and of their children:[3] and is the kingdom of the Lord Jesus Christ,[4] the house and family of God,[5] out of which there is no ordinary possibility of salvation.[6] III. Unto this catholic visible Church Christ has given the ministry, oracles, and ordinances of God, for the gathering and perfecting of the saints, in this life, to the end of the world: and does, by His own presence and Spirit, according to His promise, make them effectual thereunto.[7] IV. This catholic Church has been sometimes more, sometimes less visible.[8] And particular Churches, which are members thereof, are more or less pure, according as the doctrine of the Gospel is taught and embraced, ordinances administered, and public worship performed more or less purely in them.[9] V. The purest Churches under heaven are subject both to mixture and error;[10] and some have so degenerated, as to become no Churches of Christ, but synagogues of Satan.[11] Nevertheless, there shall be always a Church on earth to worship God according to His will.[12] VI. There is no other head of the Church but the Lord Jesus Christ.[13] Nor can the Pope of Rome, in any sense, be head thereof; but is that Antichrist, that man of sin, and son of perdition, that exalts himself, in the Church, against Christ and all that is called God.[14]I. The catholic or universal Church, which is invisible, consists of the whole number of the elect, that have been, are, or shall be gathered into one, under Christ the Head thereof; and is the spouse, the body, the fulness of Him that fills all in all.[1]