The etymology of the term religion is still a matter of controversy. The Lutheran dogmatician Hollaz writes: "Some suppose the term religion to be derived from religare (Lactantitus) others from relegere (Cicero). According to the former derivation, religion signifies the obligation rightly to worship God or something which imposes upon man obligations and duties. According to the latter etymology, religion is diligent attention to those things which pertain to the worship of God. The former derivation is more generally received." Doctr. Theol., p. 21. The Lutheran dogmatician Quenstedt cites as synonyms of religion the Greek terms θρησκεία, Jas. 1, 26; εὐσέβεια, 1 Tim. 4, 8; λογική λατρεία, Rom. 12, 1. However, none of these terms is really synonymous with religion, although each designates and emphasizes a particular phase of it. True religion is communion with the true God through faith in Jesus Christ; it is nothing more and nothing less. Still the controversy concerning the etymological meaning of religion need not trouble us, since in the final analysis the denotation of a word does not depend on its etymological derivation, but rather on its usage (usus loquendi).
However, from the common usage of the term religion we can derive no satisfactory definition of religion if we desire to include both the Christian religion and the non-Christian religions. While both Christians and non-Christians employ the term religion, each of these groups connects with it its own specific concepts and meanings, and, as we shall see, these are contradictory. The matter deserves careful attention.
Investigation shows that all heathen religions stand in direct opposition to the Christian religion. They are all, without exception, religions of the Law. To the heathen, religion means the earnest endeavor of men to reconcile the deities by their own efforts or works, such as worship, sacrifices, moral conduct, asceticism, etc. In this respect all non-Christian religions agree, no matter how much they may differ in individual details. Nor can we expect anything else; for the heathen by nature do not know the Gospel (1 Cor. 2, 6-10: "We speak ... the hidden wisdom, ... which none of the princes of this world knew"), but only the divine Law, namely, so far as this is written in their hearts. Hence all their religious thoughts move within the sphere of the Law, so that from beginning to end their religions are, and indeed must be, religions of the Law.
Christians, on the contrary, believe true religion to consist in the very opposite. To Christians, religion means true faith in the Gospel of Jesus Christ, or in the gracious message, revealed in Holy Scripture, that a perfect reconciliation has been effected between God and man through the vicarious atonement (satisfactio vicaria) of the divine-human Christ, the Redeemer of the world. Hence religion in the true sense of the term may be ascribed only to believers in Christ Jesus. And that is precisely what God's Word teaches on this point. True religion, according to God's Word, is communion with God through faith in Jesus Christ. Thus St. Paul testifies: "Knowing that a man is not justified by the works of the Law, but by the faith of Jesus Christ, even we he believed in Jesus Christ that we might be justified by the faith of Christ and not by the works of the Law," Gal. 2, 16.
Whenever theologians or entire denominations within external Christendom deny the cardinal doctrine of justification by grace, through faith in Christ, either in whole or in part, these individuals or church-bodies surrender the Christian conception of religion and adopt the pagan view. They are apostates from the Christian faith, as St. Paul declares: "Christ is become of no effect unto you whosoever of you are justified by the Law; ye are fallen from grace," Gal. 5, 4. In short, the doctrine of salvation by faith and that of salvation by works are opposites (opposita), which necessarily exclude each other, so that, if any one trusts in his works for salvation, he no longer in deed and truth professes the Christian religion.
The basic difference between the Christian religion and all other so-called religions has been aptly pointed out by Prof. Max
1. THE SCRIPTURAL VIEWPOINT OF THE CHRISTIAN THEOLOGIAN
Holy Scripture is the only source and n.orm of Christian faith and life...
3. OF THE NUMBERS OF RELIGIONS IN THE WORLD
All religions must be reduced to two classes; religions of the Law, that is, religions which endeavor to reconcile the Deity by works of the Law; and the religion of the Gospel.