Agree, this is the central thesis.
Yes it is written to believers and not some people who only superficially believed.
Post #389 points out Hebrews 10 context isn't about NT believers loss of salvation due to sin. It also cites Parable of the Sower:
Mark 4:
14 The sower soweth the word.
15 And these are they by the way side, where the word is sown; but when they have heard, Satan cometh immediately, and taketh away the word that was sown in their hearts.
20 And these are they which are sown on good ground; such as hear the word, and receive it, and bring forth fruit, some thirtyfold, some sixty, and some an hundred.
Luke 8:5 (A) A sower went out to sow his seed:
11 Now the parable is this: The seed is the word of God.
15 But that on the good ground are they, which in an honest and good heart, having heard the word, keep it, and bring forth fruit with patience.
(MY NOTE: The sower is the Holy Spirit the seed is Gods Word/Christ's Once For All time sin atoning sacrifice & the soil is the human heart).
The epistle is written to both saved & unsaved Hebrews. The severe punitive warning (vs 29) is aimed at Hebrews that had herd & understood Christ's sacrifice. Yet, they still continued to bring animal sin sacrifices to the Temple Priests, in order to avoid persecution. Note, what happened to Stephen (a true believer, Acts 7:54-60) for standing up for his faith.