“The Lutheran Church is the TRUE VISIBLE CHURCH; not the universal Church, not the only saving Church, not the only Christian Church, but the true visible Church; that is to say, the Church of the pure Word and the pure Sacraments. The invisible Church is built solely and exclusively upon the foundation of the apostles and prophets, Eph. 2, 20, and the visible Church may not forsake this foundation. The Christians, uniting for the common confession of their faith and the hearing and preaching of God’s Word, must preach and confess the pure Word. It is the only form of the visible Church that conforms to God’s will. A union of any other kind is not countenanced by God. The disciples of Christ must continue in His Word, John 8, 31, teach all things Christ has commanded them, Matt. 28, 20, hold fast the form of sound words, 2 Tim. 1, 13; Jer. 23, 28-31; Matt. 7, 15; 1 Cor. 1, 10; Eph. 4, 36; 1 Tim. 4, 16; Tit. 1, 9; 2, 1. 7; 1 Pet. 4, 11. That is a true visible Church which “continues steadfastly in the apostles’ doctrine,” Acts 2, 42. The Lutheran Church does so. Deriving all her doctrines from Scripture (formal principle) and coordinating them with the cardinal doctrine of Scripture, justification by faith (material principle), she confesses and teaches the full Christian truth. The life of the Lutheran Christians is far from perfect, but the doctrine of the Lutheran Confessions is absolutely pure. Not one of her doctrines needs correction. “I verily desire to see a true Christian council in order that many matters and persons might be helped. Not that we need it, for our churches are now, through God’s grace, so enlightened and equipped with the pure Word and the right use of the Sacraments, with knowledge of the various callings and of right works, that we on our part ask for no council.” S. A., Preface, 10. And the Lutheran Church is the only true visible Church. Those religious bodies which deny the entire substance of the Gospel, the deity of Christ and the redemptive work of Christ, are not Christian churches, but synagogs of Satan, Rev. 2, 9; 1 John 2, 23; 5, 20, 21, “outside of the Church of Christ.” Ap., I. Those religious bodies which deny fundamental doctrines of the Bible while retaining essential portions of the saving truth are, because of the believers born in their midst by the truth proclaimed in their midst, churches indeed. “We certainly must acknowledge that the Enthusiasts have Scripture and God’s Word in other articles, and he that hears it through them and believes it is saved.” Luther, XVII, 2212. But by reason of their rejection of fundamental Gospel-truths they are false, impure, heterodox churches, sects. The Lutheran Church is the only Church which teaches the pure doctrine of Scripture unmixed with rationalistic adulterations or any other form of human doctrine. That sets her apart from the others. That constitutes her peculiar glory.
“The difference between the Lutheran Church and the sects is a radical one. There are those who would classify all Christian churches, at least all Protestant churches, as sister churches, all alike possessing greater or smaller portions of the saving truth, none of them free from doctrinal aberrations. The Lutheran Church refuses to be thus classified. She is sui generis, the only true visible Church, the Church of the pure doctrine. She differs from the sects not in mere externalities, but in the matter of doctrine. And doctrine is the most important matter in the Church. The doctrine of Scripture deals with the issues of eternal salvation. Every single doctrine is bound up with these issues. Nor does the Lutheran Church differ from the sects merely in minor doctrinal points, but in fundamental doctrines, which have the most direct bearing on the issues of salvation. That is the wondrous glory of the Lutheran Church, that she has preserved and preaches the Gospel in all its fulness and purity. “This is the sum of our doctrine, in which, as can be seen, there is nothing that varies from the Scriptures.” A. C., Concl. of first part. Trigl., p. 59. That is the speech of men who stand in holy awe of God’s Word, realize the importance of the saving doctrine, and are filled with gratitude for the possession of the full truth of the Gospel. Men must not attempt to hush it with the cry of bigoted partisanship and pharisaic self-conceit. Nor by pointing to the counterclaims of other churches. The question which is the true visible Church must be submitted to the judgment of Scripture. The Lutheran Church has submitted her Confessions. “This is about the sum of our doctrine.” And she is confident that he who faithfully applies the sole standard of doctrine, Scripture, will recognize her as the true visible Church. “Although this Confession was received with disfavor by their opponents, still, thank God, it remains to this day unrefuted and unoverthrown.” F. C., Th. D., Preface, § 3.”
This quote is from pages 6 and 7 of Th. Engelder, W. Arndt, Th. Graebner, and F. E. Mayer’s book Popular Symbolics: The Doctrines of the Churches of Christendom and of Other Religious Bodies Examined in the Light of Scripture, published by Concordia Publishing House in 1934. You can download the book at https://archive.org/details/MN41551ucmf_1.