The question is the question above in the title. Or is it about apostasy?
This often misquoted verse is a subject of much debate, but what I find is that the context is often completely overlooked.
Hebrews 10:1-18 is about Christ's perfect sacrifice being superior to the sacrifice of animals.
Verse 1 is talking about animal sacrifices offered yearly being inadequate to make anyone perfect.
Hebrews 10:1 (NKJV)
1For the law, having a shadow of the good things to come,
and not the very image of the things,
can never with these same sacrifices, which they offer continually year by year, make those who approach perfect.
Compared to the single, perfect, sacrifice of Christ who perfects everyone for all time:
Hebrews 10:16 (NKJV)
14For by one offering He has perfected forever those who are being sanctified.
This is about sinning after having an animal sacrifice. There was not a sacrifice for sins until the next year under the Law. Sinning makes an animal sacrifice null and void.
Hebrews 10:26-28(NKJV)
26For if we sin willfully after we have received the knowledge of the truth,
there no longer remains a sacrifice for sins,
27but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries.
28Anyone who has rejected Moses’ law dies without mercy on
the testimony of two or three witnesses.
What all of this means is that Hebrews 10:26 cannot be true regarding those covered by Christ's sacrifice. By process of elimination, verse 26 is talking about animal sacrifices.
You may also have noticed the simplicity of how the books of the Bible are named. Romans was written to Romans, Ephesians was written to those people of Ephesus, Galatians was written to those people of Galatia, and Hebrews was written to Hebrew people.
The target audience of Hebrews are actually a mixed audience of practicing Jewish people and Jewish Christians who are being given a persuasive sermon to convert to either Christianity, reject animal sacrifices, accept the perfect sacrifice of Christ, or not revert back into the Law of Moses.