What does 'orthodox' mean, in practice? 'Orthodox' with a capital has a specialised meaning, regarding the Eastern Orthodox Church, but its uncapitalised meaning (which is not necessarily a matter of Christianity and neither the English word, nor the Greek, is found in the Holy Bible) is :


  Of, belonging to, or in accordance with the accepted theological or ecclesiastical doctrines of a particular religion.


Oxford English Dictionary

When used of Christian doctrine or Christian practice, I would expect - speaking for myself - that 'orthodox' would therefore mean the same as 'apostolic'. I would expect that what is stated by the apostles in the four gospel accounts and various epistles reliably attributed to the apostles would be what is considered to be 'orthodox' by those who follow Jesus Christ.

And it would be my expectation that each generation - including my own generation - would look back to that foundation in order to be assured as to what is 'orthodox'.

Paul says, in I Corinthians 3:10,11 :


  According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon.For other foundation can no man lay than that is laid, which is Jesus Christ.


I understand by this that Paul laid the foundation in his preaching, and we have the reliable record of his epistles. Thereafter, all others are to build (each in their own generation) upon the foundation that is already laid.

Some, in their lifetime, will build something solid which will withstand the fire. Some will heap up wood, hay and stubble, which will be burnt. Yet they themselves will be saved, albeit without a permanent, personal, legacy, I Corinthians 3: 12,13.



Is the above what Protestant churches would regard as 'orthodoxy' ?

Or have I misunderstood its application within Protestant Christianity >
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:


The Apostle's Creed
The Nicene Creed
The Formula of Chalcedon
The Athanasian Creed


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:


which eschatological system is correct
should baptism be administered to infants or those who can give a credible expression of their faith?
what is the proper church government
what is the relationship between God's sovereignty and man's free will
what is the current status of Israel


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.