Jesus describes what we might call spiritual sclerosis in Matthew 13:15. He quotes Isaiah 6:9 and applies it to the multitudes of His time. He says, For the hearts of this people have grown dull [which could mean fat or calloused]. Their ears are hard of hearing, and their eyes they have closed . . .. Their hearts, well as their other senses, had become insensitive so they were unable to receive what Christ was saying to them.
That's what the author of Hebrews is warning against - a gradual hardening of the heart, a sclerosis, that causes spiritual impairment that will ultimately be fatal because the word of God no longer has any effect.