Your problem here is that you have assumed Hebrews 6 and Hebrews 10 are about two separate groups of people - not on the basis of context clues - but on the basis of a set of theological presuppositions you have already assumed to be true. When you encounter someone like me who does not hold those beliefs you are at a loss to explain how such ideas can be derived from the passage because, in fact, it cannot be.
One of the main reasons Hebrews was written was to reiterate the great truths about Christ, His work of redemption, His unique standing before God as the Son, the sacrificial Lamb of God and His intercessory High Priesthood. The second purpose of the Book is obviously to WARN believers of the dangers of not continuing in the FAITH. The book follows this design (of TEACHING and WARNING) in both Hebrews 6 as well as Hebrews 10. Thus, it is not at all true that one has nothing whatsoever to do with the other. I personally see the book as one whole coherent work written to a particular group of believers who had a common Jewish heritage, who probably lived in the Holy Land.
Because I accept the idea that these people were believers I acknowledge both the revelations about Christ as well as the warnings contained in the book. Because of this, I do not have to subdivide chapters and assign them to various subgroups of believers TRUE and FALSE believers which the author never mentions. To me the readers and the content of this writing are one.
Since your presuppositions will not allow you accept the WARNING aspect of the Book of Hebrews since you believe salvation, one received, is unconditionally maintained you have to find a way to negate that aspect of the book altogether saying he must have been talking to some other subset of unsaved people rather than TRUE believers. However, since the author neither DIRECTLY and EXPLICITLY refers to one group and then another your interpretation seems to be drawn NOT from the book itself but rather from sources OUTSIDE the book and I think we both know who those sources are.
Here you show your willingness to engage in slander and false accusations in order to present the appearance of defending your position. Apparently, the truth or falseness of what you say is not important at all so long as you appear to be making a defense.
Where does the author of Hebrews make it clear that he is talking to a different groups in chapters 3 and 6?
You have assumed this only because of a presupposition you have imported into the word. The real test always must be can this proposition be proven using the text. Is it a common practice in the Bible that when people are given warnings by an inspired spokesman that the speaker uses as his examples, people who do not exist and could not exist?
Once again you attempt to take the mantle of a teacher who assumes they have vastly superior knowledge to common Christians, to the point that they must always be the one educating others. When questioned, such people imply that they either do not have the time to explain or that others would not understand them it if they did.
No it is not sufficient to state what you believe. The Bible says we must be willing to prove our beliefs using rational well-reasoned answers:
15 But in your hearts sanctify Christ as Lord. Always be prepared to give a (well reasoned) defense to everyone who asks you the reason for the hope that is in you. But respond with gentleness and respect, 16 keeping a clear conscience, so that those who slander you may be put to shame by your good behavior in Christ.…
(1 Peter 3:15-16)