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

1. In word, the writers of the first centuries extol the oneness of the Church to the highest degree. E.g. the Didache: "Remember, Lord, your Church … and gather her from the four winds, sanctified, into your kingdom, which you have prepared for her."¹ Clement of Alexandria: "Since God is one and the Lord is one, for this reason also that which is supremely venerable is praised, and from the fact that it is unique, as being an imitation of the principle, which is one. Into the lot of one nature, therefore, the Church is co-opted, which is one, which the heresies strive to cut into many."² Cyprian: "God is one, and Christ is one, and the Church is one, and the chair is one, founded upon Peter by the Lord's voice."³ Other Fathers proclaim the Church one mother⁴ one dove of Christ,⁵ one bride of Christ,⁶ one temple of God.⁷

2. In deed, the Christians of the earliest times truly acknowledge only one Church of Christ. Those dissenting from the union of the Catholic Church were never regarded as churches, but as sects outside the Church. Many were the sects, one the Church. Indeed, not even the sects themselves boasted that there were several churches of Christ, but each one of them maintained that it was the one Church of Christ.

Therefore, by the testimony of Christ Himself, of Christ's Apostles, and of Christian antiquity, it is established beyond all doubt that Christ instituted only a single Church. Whatever, then, the true Church of Christ may be, it is certain that it alone must derive its origin from Christ, and that apart from it no other church exists by divine right. Hence it is illegitimate to speak of "sister Churches," or simply of several churches which, although divided from one another, nevertheless all deserve the name of the Church of Christ.