The sin of unbelief

The LifeWay Leader Guide for Step 3 of “Exercise Confidence” from Hebrews 10:26-31 is a little confusing. It says the passage “likely” applied to believers, but then the commentary seems to focus on a non-believer who thinks he is saved.

To me, verse 29 indicates the passage is written to believers given the phrase “which he was sanctified.” That’s a saved person.

I’m wondering what sin the writer has in mind given the context? The Hebrew believers were turning back to Judaism, or abandoning the disciplines of their Christian faith. What sin in this context would deserve such a strong warning?

To clarify what sin the writer has in mind, I put the warnings given in Hebrews together in one statement as follows:

Do not drift away (Heb 2:1)
By unbelief that hardens your heart (Heb 3:12-14)
But God willing, go on to maturity, (Heb 6:1-3)
Because there is no more sacrifice for sin (Heb 10:26)
And there is no escape from judgment and loss of blessing (Heb 12:25)

So it seems that the sin of unbelief that hardens a heart is what the inspired writer is addressing. He has in mind a believer who is in danger of a hardened heart due to unbelief. The continual practice, or on-going sin of unbelief leads to harden heart and judgment.

Does this make sense? If so, how can we illustrate the heart-hardening nature of unbelief? I’m thinking of a “hardening catalyst”, or “hardening agent” mixed in a compound causes it to turn rock hard (as in some epoxies).

A more appropriate illustration might be to describe the way cholesterol (unbelief) leads to Atherosclerosis (hardening of the arteries).