What does Hebrews 3:18 mean?
Verse 16 mentioned rebellion as one of the ways in which our spiritual failures can result in a loss of fellowship with God. Verse 17 used the term "sin" as a more generic reference to our physical behaviors. Here, the third of four such problems is given: defiance. This is apeithēsasin in Greek, a word carrying a strong flavor of stubbornness and refusal. This, in other words, covers those moments when our sinful natures take on the form of a toddler: stamping our feet and saying, "no!"The context of these warnings is that of Psalm 95: Israel suffered for decades without entering the Promised Land, because they were stubborn and rebellious towards God. When faced with something difficult, they failed to "hold fast" to their faith (Hebrews 3:6). Christians face the same risk. The author of Hebrews is warning believers to guard their lives (Hebrews 3:12), and to lovingly encourage each other (Hebrews 3:13), to avoid this discipline.