A399 : by Tony Garland 
There are two aspects to consider in regard to the term orthodoxy: (1) the meaning of the term. (2) the content of Christian doctrine encompassed by the term.
Concerning the meaning of the term:
Orthodoxy: The English equivalent of Greek orthodoxia (from orthos [right] and doxa [opinion]), meaning “right belief,” as opposed to heresy or heterodoxy. The term is not biblical; no secular or Christian writer uses it before the second century, though the verb orthodoxein is used by Aristotle (Eth. nic. 1151a19). The word expresses the idea that certain statements accurately embody Christianity’s revealed truth content and are therefore in their own nature normative for the universal church. This idea is rooted in the NT insistence that the gospel has specific content (e.g., 1 Cor. 15:1-11; Gal. 1:6-9; 1 Tim. 6:3; 2 Tim. 4:3-4), and no fellowship exists between those who accept the apostolic standard of christological teaching and those who deny it (1 John 4:1-3; 2 John 1:7-11). The idea of orthodoxy became important in and after the second century, through conflict first with gnosticism and then with other trinitarian and christological errors. Preservation of Christianity required the maintenance of orthodoxy in these matters.1
One aspect of this definition is that it serves as a fence or perimeter around the teachings of the Bible which the historic (in our case, Protestant) Christian church has identified as a test of the boundaries of true fellowship. Those who embrace these teachings are considered brothers and sisters in Christ—who will enjoy eternal fellowship with one-another and Christ. Those who do not are considered as belonging to rival and false representations of Christianity. Variations in belief within the boundary of orthodoxy do not constitute significant enough disagreements to warrant breaking spiritual fellowship: they represent differences of belief “within the pale” of what one must hold to in order to be Christian.
Protestants generally reserve the term “orthodoxy” for the body of doctrines that are essential to the Christian faith. People who hold to this body of truth may disagree on points of interpretation on subjects that do not materially affect the central truths of God’s revelation. Differences on such matters do not make men heretics.2
Christians who are familiar with their Bibles generally have little trouble identifying views that are unorthodox — considered heretical. Such views deny fundamental claims of the Bible and therefore undermine core Christian doctrines.
However, precisely defining the content of orthodoxy can be more difficult—akin to succinctly defining the minimal elements of the faith what one must believe in order to be saved (a subset within orthodoxy).
I've found the following helpful:
Any list of the essential Christian doctrines that constitute orthodoxy must include the following: (1). One personal, eternal God who exists necessarily in the Trinity of His sacred persons. (2). The inspiration and authority of the Bible as the very word of God, the sole source of Christian doctrine. This includes the historical integrity of the OT and the NT, including their record of the supernatural interventions of God in the affairs of men. (3). God’s creation ex nihilo of the heavens and earth and all that is in them, and His providential sovereign control of His entire creation. (4). The Person and work of Christ, with particular emphasis on the following: His essential and eternal deity; His incarnation and virgin birth; His sinlessness and obedience to God in His life; His once-for-all vicarious atonement by the shedding of His blood at Calvary; His actual bodily resurrection from the dead; His ascension to God’s right hand to be the great high priest and advocate of His people; His second coming. (5). The essential and eternal deity and the Trinitarian personality of the Holy Spirit. (6). The historicity of the biblical narrative of the creation of man, his fall into sin, God’s consequent curse of the earth, and the corruption of the entire human race by Adam’s fall. (7). Salvation by free grace without any addition of human merit at any stage in the work. This includes the following truths: the depravity of all men by nature and their inability, while unregenerate, to produce spiritual good; the necessity of the Spirit’s work to regenerate souls dead in sins and to create repentance and faith in them; the full application to God’s people of the redemption purchased by Christ; eternal life as the free gift of God, to be received by faith, not obtained with money or merit; justification by faith alone, including the forgiveness of sins and the imputation of Christ’s perfect righteousness as the ground of the believer’s acceptance with God; the union of all believers with Christ as members of His body, the church; sanctification by the power of the Holy Spirit, who works in those whom God has justified to produce good works as the evidence of a living faith; glorification with Christ in the blessed enjoyment of eternal life. (8). The resurrection of saints and sinners, God’s judgment of all men, followed by the everlasting blessedness of the saints (heaven) and the everlasting punishment of the lost (hell). Some may add their own denominational distinctives to the list, but this would be a mistake. Anyone who holds these truths will never go wrong on anything that is essential to the saving of men’s souls. All who are within this framework of fundamental truth hold to orthodoxy, despite all their minor variations of views. All who are outside of this framework are heterodox, whatever other points of interpretation they may hold in common with the orthodox.3
Blessings - Tony
Endnotes:
Sources:
| Ref-1363 | Alan Cairns, Dictionary of Theological Terms: Expanded Third Edition (Greenville, SC: Ambassador Emerald International, 2002). ISBN:1-889893-72-2a. | | Ref-1594 | Daniel J Treier, ed., Walter A. Elwell, ed., Evangelical Dictionary of Theology (Grand Rapids, MI: Baker Academic, 2017). ISBN:9780801039461b. |
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