What does Hebrews 12:17 mean?

This verse completes a reference to Esau, the older brother of Jacob (Genesis 25:24–26). Esau was entitled to certain privileges as the firstborn, but he treated that status with recklessness and contempt. As a result, Esau lost out on those blessings. The prior verse referred to his attitude as "unholy," a serious charge. The key event demonstrating Esau's attitude towards his birthright was mentioned in the last verse, a story given in Genesis 25:29–34. There, Esau carelessly promised to sell his birthright to his younger brother, in exchange for a bowl of soup.

Here, the end result of Esau's mistake is explained. The prior verse used the Greek term bebēlos, also used to describe something "profane." Esau learned, the hard way, that he could not show contempt for his blessings, then expect to actually obtain them. When the time came to inherit his full blessing, Esau found it had been given to someone else (Genesis 27:34–35). In the context of this passage in Hebrews, this is meant to reinforce prior warnings. Earlier verses mentioned an Old Testament metaphor of a "bitter root;" this referred to a person who presumed they could sin against God without suffering the consequences (Hebrews 12:15; Deuteronomy 29:18–19). God does not take sin lightly (Hebrews 2:1–3), and He is especially offended when those who claim to know Him choose to ignore His will (Hebrews 10:26–31). Christians cannot expect to treat our relationship with Christ in a careless way without consequences.