Hebrews 10:26 can be a scary verse. One woman I know has been crippled by the fear that she is damned because of ongoing sin. Whenever I declared the gospel to her and held out the grace of Jesus she would always come back to these verses:
If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. (Heb 10:26-27)
Whatever happens we don't want to be found to be ones who "deliberately keep on sinning." So what does it mean?
Different things can be said, but let me just take up one line of inquiry. Verse 26 uses 'sin' in two senses - first as a verb, then as a noun. It's interesting to note that Hebrews only uses 'sin' as a verb twice. But 'sin' as a noun is everywhere. Here are all its uses:
Sin as a noun:
is purified - 1:3
is atoned for - 2:17
is not remembered anymore - 8:12; 10:17
is put away once and for all - 9:26
is borne by Jesus once and for all - 9:28
Christ is sacrificed for it once for all time - 10:12
Christ is without it - 4:15
is dealt with in shadowy way by High Priest / old covenant - 5:1,3; 7:27; 10:2,3,4,6,8,11,18,26; 13:11
hardens and deceives - 3:13
gives fleeting pleasures - 11:25
easily entangles - 12:1
causes struggle - 12:4
Sin as a verb is only mentioned twice:
Israelites ‘sinned’ and their bodies fell in the wilderness - 3:17
Deliberately sinning – no sacrifice for sins remains - 10:26
Sin (noun) – has been purified, atoned for, put away and borne in the sacrifice of sinless Jesus once and for all. It is therefore remembered no more. This is precisely what the old covenant promised through its shadows but never effected itself. Sin remains a reality for the Christian – it offers fleeting pleasures. But it deceives and hardens, it easily entangles and causes painful struggle.
To sin (verb) – is a decisive and deadly rejection of the Lord. The Israelites “sinned” in the wilderness and so they died (3:17). This is the verdict upon 40 years of their constantly wayward hearts. They did not want the Lord and His future and so He swore that they would not enter His rest. People today ‘deliberately sin’ when they reject Jesus, their one Sacrifice for sins and Forerunner to glory. If they forsake Him, no sacrifice for sins remains. (10:26)
I wonder therefore whether the slight overtranslation of 10:26 in most versions ("Deliberately keep on sinning ...") spotlights the wrong thing. The unforgiveable nature of this kind of sinning is not really its ongoingness - though it is an ongoing attitude. The unforgiveableness of this sin is that it is a rejection of the very One in Whom forgiveness is offered. The author is not telling us: "a spot of occasional sinning is alright but ongoing sinning is damnable." He's saying that sin is put away by Christ once and for all, but the person who rejects Christ deliberately has nowhere else to turn.
Hebrews is written to a people always tempted to trust the shadows and not the Substance. They look to angels and Moses and temple and priests and goats and bulls and everything but Jesus. But Jesus is the One we are to See and Fix our thoughts upon, etc, etc. If we have seen HIM and then turn away again to worthless sin-bearers - no sacrifice for sins remains. We've rejected the one Life-raft.