score:2
The only orthodoxy that counts for anything, with regard to matters of faith in God, is what God considers to be orthodox.
As the Apostles of the first century A.D. were inspired of the Holy Spirit to write what they wrote in the Christian Greek scriptures, that would reveal to Christians of every succeeding generation what the orthodox foundation of faith was - Jesus Christ, crucified and risen, to return in glory, the builder of his Church. I reckon all Protestant churches would agree with that in theory. The problems come with practice.
However, there is a general consensus within Protestantism as to what beliefs and practices became unorthodox. Many books have been written about Church history, detailing where groups developed that could no longer be considered orthodox. I was at a Protestant lecture on Church history yesterday where the following points were made, that may help clarify any answer to your question.
Two types of heresies arose, Legalistic (e.g. the Nazarenes, the Ebionites, the Essenes) and Philosophical (e.g. Gnosticism. Docetism, Manicheanism, Neoplatonism). Heresies need to be distinguished from errors, however. Errors arise from misinterpretations and over-emphases. When the errors are not to do with the foundational doctrines of Christianity (the actual gospel about the person of Christ) then grey areas are entered into where it becomes more difficult to say whether the groups in question are unorthodox or still fit to be considered within mainstream Christianity, or not.
Then schisms also need to be distinguished from heresy. This is where schism arises on matters of discipline and/or ritual, e.g. the Easter controversy (about the correct date for celebrating Christ's resurrection) and Donatism (where Donatus wanted to exclude from the Church Caecilian of Carthage as a traitor who had apparently renounced his faith due to the horrific persecution of Diocletian.)
It would require a person very learned in the history of Protestant history to give a categoric answer to your question. I am not that person. However, I offer the above comments in the hope that they might help a little to open up the complexity of this subject.
Upvote:2
I think your suggestion -- apostolicity and grounded in scripture -- is a good start, and in fact sounds to me a lot like Sola Scriptura.
Answer 1. The Solae
That thought led me to Wikipedia, which says the Five Solas are the founding principles of the Reformation... so to me, that sounds like at least part of the protestant definition of orthodoxy.
https://en.wikipedia.org/wiki/Five_solae
I'd add that the Solas leave room for deepening the faith -- for instance, the Trinity was, is, and ever shall be Good Theology.
And that leads me to the last point: the solae are self-based reforms. Church history and tradition is not thrown away with the garbage; rather, it is subjected to Scripture. Presumably, the chaff is sifted from the grain. God willing, it would be done increasingly, gently, and motivated by love for God and one's neighbor.
Answer 2. Paleo-Orthodoxy
THAT reminded me of "paleo-Orthodoxy", a protestant theological movement in the United States which emerged in the late 20th and early 21st centuries and which focuses on the consensual understanding of the faith (Regula Fidei, probably) among the Ecumenical councils and Church Fathers.
Being a protestant thing, I assume it still views the early church through the lens of the Five Solas.
Upvote:5
Overall, orthodoxy is pretty loosely defined. You may well find protestants (again, a very wide group) to come to different conclusions. But I think in general orthodoxy is understood as teaching which falls within the bounds of what has always been considered the Christian faith. So then what falls in that range? It is really the content of some of the great definitional statements of the church:
Within these statements there is much defined, but also much left undefined. For example, you will search those statements in vain trying to answer questions of:
Many other questions could be added to that list. The idea is that you could have, say, a Calvinist who believes in infant baptism and covenant theology who has strong disagreements with a dispensational Arminian who only baptizes believers, but they could both acknowledge one another as brother/sister in Christ within the bounds of orthodoxy.
I hope that helps.