Because “no sacrifice for sins is left.” This is obviously the parallel of 10:18, which says that once we’ve been saved, “there is no longer any sacrifice for sin.”
You see, Jesus died but once. We enter with him into his death but once — at baptism. We are then resurrected with him — and can no longer re-enter his death.
If we remain true to our faith and our penitence, we never need to find a second forgiveness. We were saved “once for all” and were made “perfect forever.” But if we rebel against Jesus, reject him as Lord, and repudiate the penitence we had at the beginning, we’ve —
* become an enemy of God
* trampled the Son of God under foot
* treated as an unholy thing the blood of the covenant
* insulted the Spirit of grace
We can’t accept God’s blessings while refusing to even try to live as God wishes. We can’t enjoy the benefits of Jesus’ sacrifice by entering into his death without also entering into his resurrected life.
The author follows this passage with a series of parallel exhortations —
(Heb 10:35-36) So do not throw away your confidence; it will be richly rewarded. 36 You need to persevere so that when you have done the will of God, you will receive what he has promised.
(Heb 12:15) See to it that no one misses the grace of God and that no bitter root grows up to cause trouble and defile many.
The second-quoted verse is often overlooked, but it’s a powerful passage. It refers back to this warning —
(Deu 29:18-20) Make sure there is no man or woman, clan or tribe among you today whose heart turns away from the LORD our God to go and worship the gods of those nations; make sure there is no root among you that produces such bitter poison. 19 When such a person hears the words of this oath, he invokes a blessing on himself and therefore thinks, “I will be safe, even though I persist in going my own way.” This will bring disaster on the watered land as well as the dry. 20 The LORD will never be willing to forgive him; his wrath and zeal will burn against that man. All the curses written in this book will fall upon him, and the LORD will blot out his name from under heaven.
The great danger is in “cheap grace,” that is, allowing grace to become an excuse to sin. You see, that’d be to deliberately keep on sinning. Therefore, if you think, “I will be safe, even though I persist in going my own way,” you fall away.
The teaching of Hebrews on the dangers of falling away are not new. God is gracious and forgives sin — but not for those who refuse to join in the life of Jesus. You can throw away what God has given you.
Now, back in 6:4-6 we were told that the danger of falling away is that we become so hardened that we never repent. Here we’re told that the danger is that we can’t again reach Jesus’ sacrifice. And these are surely the same thing seen from different angles.
Therefore, if someone repents, he never fell away. Indeed, if he repents, he certainly has forgiveness, and therefore is not described in Heb 10:26ff. Now, this tells us that God is far more patient with us than we sometimes imagine. The Parable of the Prodigal Son is one of my favorite examples of God’s love — a love so great that he will put up with some very wicked behavior before finally giving up on us.
And so we have this paradox. It’s both easy and difficult to fall away. It’s easy in that we only need to develop a hard heart. That’s all it takes. It’s not that we committed particularly awful sins. It’s that our heart has become rebellious.
But it’s difficult because God has given us his Spirit and each other to help us make it to the end and to call us to repentance when we mess up. It’s difficult because God can be remarkably patient with his beloved children. And it’s difficult because God will never reject someone who comes to him with faith and penitence.
However, the moment we decide that we can take advantage of God’s patience and sin intentionally with impunity is the moment we put ourselves in truly grave danger. And — much worse — when we turn in that direction, we begin to silence the Spirit and our conscience. We’ll likely also withdraw from our Christian brothers and sisters. And over time (and sooner than we wish), it’ll become harder and harder to turn back to God.
Eventually, it becomes so hard to turn back that our consciences become seared as with a hot iron. And then we’re irretrievably damned.
Of course, this means that the rest of us have an obligation to our brothers and sisters to help them make it and so to turn them from their hardheartedness. And we are routinely too slow to begin and too quick to stop. We give up, I believe, long before God does, and we don’t even make the effort to help until they are well on their way to damnation.
The solution is for us to expressly give one another permission to meddle — to call us to account, and even to pry a bit. We Westerners are proud of our privacy and our right to keep others out of our business. And that’s why far too many fall away and no one else even tries to prevent it.
We think it’s not our problem. We are not our brother’s keeper. But we are. If we make it to heaven, it’ll be because we were surrounded by a horde of supporters, cheering us all the way.
Think of it like athletics. No athlete achieves his true potential without a coach, and no coach worth his salt does his job without invading the privacy of his athlete — telling her what to eat and drink, how much sleep to get, when to show up for practice — providing a level of discipline most of us cannot do for ourselves.
And in Christianity, we don’t have coaches. We have churches.