It is God's will and command that all believers should hear, learn, and proclaim only the pure Word which He Himself has given them in Holy Scripture, Jer. 23, 30-32; 1 Tim. 6, 3-5. All who pervert the Word of God and teach their own doctrine in place of God's revealed truths, Matt. 15, 9, Scripture condemns as false prophets, Matt. 7, 15, teachers, both of fables and of damnable heresies, 1 Tim. 1, 3-7; 2 Pet. 2, 1, ungodly men, Jude 3. 4, men of corrupt minds and reprobate concerning the faith, 2 Tim. 3, 1-8, deceivers and antichrists, 2 John 7, whom all Christians must avoid, Rom. 16, 17; 2 John 10, as conceited ignoramuses, 1 Tim. 6, 3-5, who are under God's curse, Gal. 1, 8, etc. As Holy Scripture condemns all sinful conduct, 1 Cor. 5, 9-11; Rev. 21, 8; 22, 15, so it likewise condemns every corruption of the Christian faith, as this is set forth in God's Word, Gal. 3, 10; 5, 10-12; Rev. 22, 18. 19; Matt. 18, 6. 7; 5, 19.
All churches that tolerate and follow such perverters of the divine truth are known as heterodox churches (ecclesiae heterodoxae, ecclesiae impurae), while churches that teach the Word of God in its truth and purity and administer the Sacraments according to Christ's institution are orthodox, or pure, churches (ecclesiae orthodoxae, ecclesiae purae).
This distinction all sincere Christians must carefully observe, especially at this time, when the spirit of indifferentism is alarmingly spreading in so many churches and there is a manifest tendency to discard the Christian doctrine (abrogation of creeds) and to reorganize the churches on a "broader basis of social service" (applied Christianity; the gospel of social service). The program of present-day Modernism calls for the abolition of every confessional basis and is therefore in opposition to the direct commission which Christ gave to His Church, Mark 16, 15; Matt. 28, 20; 10, 32-39.
In order that believers may rightly distinguish between orthodox and heterodox churches, the following points must be kept in mind.
In the first place, a church is orthodox, or pure (ecclesia vera seu pura), not simply when it acknowledges the divine truth in general through confessions which are in accord with Scripture, but when it actually teaches the divine truth without qualification and prevents or suppresses all error. In other words, a church must be pure, or orthodox, not only in principle, but also in practise, so that it earnestly reproves and disciplines all who teach false doctrine. But this is not all. If a church wishes to be truly orthodox, it must not only teach in conformity with Scripture, but also insist upon a practise that is in strict accord with whatever the Word of God inculcates (conditions of church-membership, attendance at Holy Communion, regulation of the Christian life of its members, opposition to religious indifferentism and unionism, etc.). A church which is orthodox in theory only, but not in practise cannot be considered as being truly orthodox, since it disregards Holy Scripture in its insistence upon the proper application of the divine truth to life, 1 Cor. 5, 13; 11, 20-22; 6, 1-6; 14, 34-40; etc.
In the second place, an orthodox church does not become heterodox (ecclesia corrupta) through errors which accidentally occur in its midst; for it is only the toleration of false doctrine and an unscriptural practise that make a church heterodox. Even in the churches established by the apostles, errorists arose, who succeeded in spreading false doctrines, Acts 20, 30; Gal. 1, 6-9; 1 Cor. 15, 1-19; but they were reproved and condemned.
Hence a church becomes impure, or heterodox, only when it ceases to apply God's directions given in Rom. 16, 17; 2 John 10. 11; Matt. 18, 17; etc., and so permits error and ungodliness to exist side by side with truth or even to gain the ascendency over truth.
Heterodox churches are sects, inasmuch as they adhere to error, which separates them from the Church. "Ecclesia quatenus impura, non est ecclesia." They are churches inasmuch as they still adhere to the Christian truth and are thus joined to the Church, provided, of course, their errors do not deny the fundamental articles of the Christian faith, in which case they cease to be heterodox churches and become antichristian cults (cp. Calvinism with Unitarianism; 1 Tim. 6, 20.21; 2 Tim. 2, 16-18).
Although heterodox churches, in the common sense of the term, still adhere to the fundamentals of the Christian faith and so harbor true believers in their midst, nevertheless all Christians who recognize their error are in duty bound to sever their connection with them, since, on the one hand, one error is bound to produce others, Gal. 5, 9, and, on the other, the toleration of a recognized falsehood is a denial of the divine truth, Matt. 10, 32-36, and this is incompatible with the true Christian profession, Rom. 16, 17; 2 Cor. 6, 14-18.