The Lutheran catechism says: “The church is holy Christianity, or an assembly of all believers, under Christ, their chief, in which the Holy Ghost through the Gospel and the sacraments offers, communicates, and secures divine salvation,” meaning, by this, that the Catholic Church has gone astray and has fallen away, and that the true tradition is preserved in Lutheranism.
For the Catholics the divine church coincides with the Roman hierarchy and the Pope. For the Greek Orthodox the divine church coincides with the establishment of the Eastern and the Russian Church. For the Lutherans the divine church coincides with the assembly of men who recognize the Bible and Luther’s catechism.
Speaking of the origin of Christianity, men who belong to one or the other of the existing churches generally use the word “church” in the singular, as though there has been but one church. But this is quite untrue. The church, as an institution which asserts of itself that it is in possession of the unquestionable truth, appeared only when it was not alone, but there were at least two of them.
So long as the believers agreed among themselves, and the assembly was one, it had no need of asserting that it was the church. Only when the believers divided into opposite parties, which denied one another, did there appear the necessity for each side to assert its authenticity, ascribing infallibility to itself. The concept of the one church arose only from this, that, when two sides disagreed and quarrelled, each of them, calling the other a heresy, recognized only its own as the infallible church.
If we know that there was a church, which in the year 51 decided to receive the uncircumcised, this church made its appearance only because there was an other church, that of the Judaizing, which had decided not to receive the uncircumcised.