Like you replied to another post, there IS only ONE truth regarding the gospel. Scripture teaches, and the assembly of believers uphold and support, that we are saved by the grace of God through faith and not of our works. Do some denominations teach we are saved by grace AND works? Yes, they do and they did - even in the Apostles' lifetime. Does that negate the truth of the gospel? No. It doesn't change, it has ALWAYS been by grace through faith apart from works. People have always tried to pervert the gospel and they are refuted BY Scripture.
No matter what a church/assembly calls itself, if it perverts the gospel, it cannot be the bulwark of the faith, can it? How do we know if a congregation teaches and believes the gospel? Look at what they preach and compare it to Scripture - which is the authority ALL of us should be measured against. On the subject of OSAS/Perseverance of the saints VS sin committed after belief can lose your salvation should be decided based upon who has the better Scriptural argument as well as what was the historical belief of the Christian church. The church "fathers" can be looked at to see what they professed, but they are still bound by Scripture, which is the ONLY infallible and objective source we have been given by God.
At one time, even the Roman Catholic Church believed in justification by faith apart from works. You can look at the writings of Ireaneus, Origen, Athanasius, Jerome, Augustine and others to see that they were pretty straight on the gospel. Over time, though, we can see subtle changes creep in and a departure from that simple truth as well as additional dogmas introduced that had no ancient witness nor Scriptural warrant. A big reason for the Reformation was the Catholic Church's negligence and perversion of the gospel and justification by faith AND works became the dogma and Christians who believed in the gospel of Scripture were persecuted as heretics when only a few centuries previously the heretics were the ones who taught what the Catholic Church now espoused. The Reformers wanted the church to RETURN to the ancient faith.
So, getting back to your question, the various denominations under the umbrella called "Protestant" at one time MAY have believed and preached the identical gospel to the Apostles and the early church. Where they have remained faithful to that, they continue to support the truth. Where they have strayed away from it, they cease being the "church of the living God". It's no more complicated than that.
The "diametrically opposed" theologies you claim disqualifies any Protestant denomination from equality with "your" church is really a smoke screen. That's because it doesn't matter what a church calls itself, it isn't a church of Christ if it preaches an accursed gospel. There CAN be differences on non-essential doctrines and there's nothing wrong with that as long as the gospel remains faithful to Scripture and the major doctrines that accompany the gospel are believed. The theology of once saved/always saved (OSAS) is the true gospel because it acknowledges that we can do nothing to merit the grace of God that brings salvation. Since we do not earn our salvation by our deeds, then we sure don't have to do deeds to "keep" it. It is grace all the way to heaven. THAT is the truth and believers must agree on that to be Christians. Belonging to a church is not what saves. It is receiving and trusting in Christ who saves us by his blood. The gift of God is eternal life through Jesus Christ our Lord.
The issue is how do we determine determine Truth. Under the Roman model the magisterium which has oversight over Divine revelation infallibly defines what it is and means, and who is a faithful teacher, thus those whom it rejects are to be rejected. But which nukes the church. For if this is the case, how could anyone follow an Itinerant Preacher who reproved by Scripture (upon which He established his own claims) the very men who sat in the seat of Moses, and whose authority they thus challenged and rejected? (Mk. 7:2-16; 11:27-31)
While the office of the magisterium is valid and is to have authority, yet it is not assuredly infallible, and under the NT its authority is to be established and maintained by Scriptural probity and power.