The overall message of Hebrews is to keep moving forward, don't turn back. The writer to the Hebrews is doing this by showing them who Christ is. They're wanting to go back to Moses. Thus far in the book, he's already shown them that Jesus is far superior to Moses. Moses was a great servant, who was faithful in all of God's house, but Christ is the Son, who is faithful over all of God's house. Moses revealed what Christ himself would do, that what Moses spoke was ultimately fulfilled in Christ. We don't want to go back to the prophecy that looks forward, we want to go to the thing itself, the thing that Moses foreshadowed.
The writer of Hebrews tells them that if indeed they turn back then it's only an indication that their faith was never genuine. He says, you should fear. Fear lest there be any among you, someone there with an evil heart of unbelief. The sin that we're to fear greatest is not that we might fall into some physical sin as great as that is, as serious as that is. It's the sin of unbelief, that is the great sin against God. It's for this sin that Jesus said it would be worse on the Day of Judgment for those villages that saw Jesus and heard him preach, than for Sodom and Gomorrah. Because he said, "If the works that I did in you had been done in Sodom and Gomorrah, they would have repented."
So the great fear is that we would hear and not believe, not repent, not persevere and trust in Christ. But now with this admonition, the writer has this deft hand in which he's not just telling us the negative. He's not just saying, "Oh, you need to be afraid of this." At the same time, there comes along this encouragement.
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