This next part seems like it is resisting being written! It's tempting to blame spiritual opposition, but owning up to my own limitations is simpler and more realistic. After “sleeping on it", I realized that there is some foundational material I was going to mix into discussion of particulars that would more simple and more clear if I wrote about it up front.
One thing is to recognize that there is a lot of ground between full acceptance and clear rejection of an idea. Sometimes people simply haven't heard that idea. A new believer or one who hasn't received a lot of Christian teaching might be in such a position. Or a person might be in the position of having heard of an idea, but not enough to have a satisfactory understanding, and therefore not have committed for or against the idea and the implications thereof.
Then there's boundaries? Boundaries of/to what? To salvation? Ultimately, I cannot have the knowledge to say with certainty, “No, ______ is not saved.” God has that knowledge of the human heart, I do not. On the other hand, there are some real things, ideas rejected or ideas embraced that a Christian believer can see in another person that point toward a person probably being lost or probably being saved. How about fellowship? Again, a simple, “Yes,” or, “No,” doesn't work. One may not know or understand a certain Christian teaching – even a fundamental teaching – and still be a believer with whom I might or should share fellowship or minister in some areas. “Fellowship” means to share things in common; “ministry” means service. Two Christian believers of very different background – e.g. a Catholic and a Pentecostal – have their faith in Jesus and the Great Commission (to bring others to faith in Christ) in common. There's a lot of room for fellowship and service together with “just” those things in common! That fellowship and ministry will, however, very likely lead to areas of difference being discussed. Having recognized each other as fellow believers, having shared fellowship, having served side by side, such discussions will very likely be of a different tone than a similar discussion between two strangers who know little of each other. And such discussions between two believers who have shared fellowship and have served side by side are more likely to improve both believers' understanding of the Christian faith. On the other hand, there are practical boundaries or limitations in such fellowship or service that can result from differences in understanding of Christian teaching. Recognizing and respecting those limitations should not mean avoiding certain other believers entirely, however!
In Part 2, I suggested that I might recall further Scripture passages that point to essential or boundary teachings. One such Scripture occurred to me last night:
But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ. For if someone comes and proclaims another Jesus than the one we proclaimed, or if you receive a different spirit from the one you received, or if you accept a different gospel from the one you accepted, you put up with it readily enough. (2 Corinthians 11:3-4, ESV)