As the context shows (Hebrews 10:23), the author of Hebrews was concerned here, as throughout his letter, with the danger of defection from the faith. Most sin is "deliberate," but the writer was influenced here by the Old Testament's teaching about sins of presumption (Numbers 15:29‑31). This kind of sin lays outside the sacrificial provisions of the Law. There is no sacrifice for this kind of offense towards God.

Apostasy from the faith is such a "willful" act, that those who commit it have renounced the sacrifice of Jesus that was provided for them. This leaves them with no sacrifice to hold back the wrath of God from them (Hebrews 10:31), no sacrifice for sins is left. If the sacrifice of Christ should be renounced, there remained no other available sacrifice which could shield an apostate from God's judgment by raging fire (Hebrews 10:27).

A Christian who abandons "the confidence or faith [he] had at first" (Hebrews 3:14), puts himself on the side of God's enemies, as the writer of Hebrews has said (Hebrews 10:27). The apostate (one who gives up or abandon's his faith) in effect is "crucifying the Son of God all over again and subjecting Him to public disgrace" (Hebrews 6:6).
Such reprehensible conduct can scarcely be worthy of anything but God's flaming indignation and retribution (Hebrews 10:27).
Under the Old Covenant, if an Israelite spurned the Mosaic Law and at least two or three witnesses verified his actions, he was put to death. If defiance of an inferior covenant could bring such retribution, what about defiance of the New Covenant which, is far superior? The answer can only be that the punishment would be substantially greater in such a case. And what could be greater than physical death? Spiritual death.
In Hebrews 10:29 (NIV) the words "sanctified him" refer to true Christians. Already the writer to the Hebrews has described them as "made holy (Gr. 'sanctified') through the sacrifice of the body of Jesus Christ once for all" (Hebrews 10:10 NIV) and as "made perfect forever" through this sanctifying work (Hebrews 10:14 NIV). Some seek to evade this conclusion by suggesting that Christ is the One referred to here as "sanctified" or that the person only claims to be sanctified. But these efforts are foreign to the writer's thought and are so forced that they carry their own refutation. The author's whole point lies in the seriousness of the act. To treat "the blood of the covenant" (which actually sanctifies believers) as though it were an "unholy" (koinon, "common") thing and to renounce its efficacy, is to commit a sin so heinous as to dwarf the fatal infractions of the Old Covenant. To this, an apostate adds the offense of insulting the Spirit of grace who originally wooed him to faith in Christ. This kind of spiritual rebellion clearly calls for a much worse punishment than the capital penalty that was inflicted under the Mosaic Law. (The Bible Knowledge Commentary pages 805-806)

If we deliberately, willfully, and defiantly go back to a life of sin this could happen: Your heart could be hardened, causing unbelief and departure from the Living God (Hebrews 3:12).

Why is sin so dangerous? It is linked to the atonement, the sacrifice of Jesus. To turn back willfully to sin, is to disregard the work of God's grace, provided for you by Jesus on the cross. If you renounce the sacrifice of Jesus and His Person, there is no provision of salvation for you.

The Apostle Paul told Timothy,
Beware, be on watch that this does not happen to you!

And don't let modern education (called philosophy, Colossians 2:8) deceive you, and take you from the Living God (Romans 1:22, 25).