Bluegoat;8292831:
So they are unaffiliated Christians. They do not belong to any group beyond the people with whom they spend Sunday’s. There is no association beyond thie immediate congregation, but they belief Protestant doctrines of one form or another, whatever seems to make sense locally.
Well, they would say they belong to the church universal, as instantiated within their own local group. So if they were really living out their congregational philosophy, they would understand all Christian groups to be part of the same body, but each a kind of organ. It would actually be very similar to the way the Catholic Church understands individual parishes to all be part of the one Catholic Church. They don’t though see the unity being created by an external political structure - rather, they see the unity being through faith in Christ.
Is it immpossible to tell what a given local simply Christian group believes about anything then, other than they are unaffiliated, not part of a greater body?
Not impossible - you could ask them.

Which is actually part of the point from their perspective - you have to engage with the community. But the term non-denominational in itself doesn’t say a lot. They do come out of a particular tradition of teaching, so it is possible to make some educated guesses.
But what about Jesus’s promises to send the Holy Spirit to be with His Church, never abandon her, lead her into ALL truth, bind in heaven what she teaches on earth?
I don’t see that this promise is actually very specific though. For one thing, what makes you think differences in what are considered non-essential teachings mean that the Holy Spirit has abandoned them? And when is it that we will be led into all truth? In the Church militant? Most of these groups would probably say that this is to be expected in the Church Triumphant, but not before.
This is actually a little difficult for a Catholic to argue to a non-denominational, because on the face of it, all groups have erred and even taught things falsely or in a confusing way. And it is pretty difficult to prove that isn’t the case. And even in the Catholic Church there are questions left unanswered (the status of particular personal revelations, for example, or whether evolution is true.) So it seems obvious to such a person that on Earth, no Church has ALL truth, and no church it totally free of error. So they understand the promise to be saying that the Holy Spirit will prevent the Church Militant from falling into error on essentials.
This is true. Most Protestants agree on this. Who defines what a basic essential is? Wouldn’t it seem that a doctrine on salvation is essential? If some say salvation can be lost and others say no matter what sin a person commit, once they have “been saved”, accepted Jesus as their personal savior, or whatever other non-scriptural formula, they can never lose salvation even if they murder, rape and torture. I know Protestants who believe in transubstantiation, or consubstantiation, other say it is all symbolic. The claim they agree on the basics is simply not true, unless the basics is a statement that the Bible is the word of God.
I think most would say, for example, a belief in salvation through Jesus Christ is essential; a theory about just how that works is not essential. A theory is just our attempt at an explanation of how it works, and to think it is essential to our salvation is incorrect. A lot would say the essentials are belief and acceptance of Christ and our salvation in and through him; belief in the Bible as the inerrant word of God which contains everything required for salvation; and belief in the Trinity.
Secondly, if the only things they dispute are nonessential things that don’t matter that much, that constitute their differences, then they took pretty drastic action over them. They disobeyed Christ’s unity command and split themselves into thousands of denominations over petty matters.
Well, since non-denominationals are congregationalists, this argument is likely to fall flat with them - they think the Church is meant to be instantiated in many local congregations. Although they would agree with you that denominations are a bad thing, they would also think the claims of the Catholic Church and its political structure is also wrong. They would support a movement from what they would see as false claims of unity to a model where every congregation is self-governing, and they would probably see the move away from the Catholic model to denominations as an attempt to move in that direction.
I think what was said is the origins of relativism come from Protestant thought. The nondenominational phenomenon certainly reflects relativism. They believe whatever their local folks teach, but maybe are not right about it, and decide for themselves what they believe. Protestantism descends into relativism, because it lacks any mechanism to discern truth. The Bible alone doctrine most Protestant’s hold is what makes this descent inevitable as the history of denominationalism reveals.
No, I think relativism was around long before Protestantism (man is the measure of all things), and some Protestants are much more strict about the objectivity of truth than Catholics are - that is where you get Biblical literalists and fundamentalists.
Now, I think what you are talking about here is not so much relativism as individualism, which may have its origins in Catholicism rather than Protestantism IMO, with the development of humanism and perhaps the move to a system with an individual at the top. But it is of course always hard to pin a whole movement on a single cause like this.