God warns the saints to not sin willfully: He threatens severe chastening if we do. (He 10:26-27) What types of sins does this include? How do we avoid committing them?
The Greek is Ἑκουσίως (Hekousiōs), appearing only here and (1 Peter 5:2); it means deliberately, willingly, as opposed to thoughtlessly, instinctively, or from ignorance, weakness or under duress. It modifies the Greek ἁμαρτανόντων (hamartanontōn), to go on sinning. The thought is that the sinful action is habitual, premeditated, intentional, brazen, defiant … knowing the law of God and despising it. (Ro 1:32)
Biblical examples would include the sanctimonious lying of Ananias and Saphira, claiming to donate all the proceeds from the sale of their land while they were keeping back some for themselves (Ac 5:1-2), who were immediately and supernaturally slain. (Ac 5:5,10) The Corinthian who took his father’s wife (1Co 5:1) was delivered over to Satan by the church for the destruction of his earthly body so his spirit would be saved (1Co 5:5), and a man gathering sticks on sabbath (Nu 15:32) was promptly stoned to death. (Nu 15:35-36)
The context of God’s warning refers back to the precedent He sets in Torah: anyone in Israel caught despising Torah would be executed without mercy. (He 10:28) Mercy was available for those who sinned ignorantly (Nu 15:27-29), but there was no pity for those despised Torah and sinned presumptuously. (30-31)
If we find this harsh, inconsistent with the New Testament god of love and mercy, we’re trusting in another Jesus, one not found in scripture: the punishment for believers who sin willfully is not less severe but more. (He 10:29) Torah’s punishment was carried out by civil authority, but the punishment of believers is designed and carried out by God Himself and may very well be much worse than death. (30) Don’t go there; it won’t be worth it, not even close. (31)
David’s adultery with Bathsheba would certainly also fall into this willful category (2Sa 12:9); he didn’t die for it, but he may often have wished he had, for all the suffering and tragedy which followed because of it. (10-12)
It isn’t cruelty that drives God’s severity; God is good; there’s no malice in Him. God’s love moves Him to severity as appropriate. (Ro 11:22) The consequences of sin are simply too devastating to be left unchecked (Mt 5:29-30); God loves the saints way too much to let us go off and destroy ourselves and others. He will do whatever is needful to bring us back and keep us close because He loves us. (He 12:5-6)
When we’re tempted to sin presumptuously, we can ask God to keep us back from it and restrain us. (Ps 19:13) We can also assure ourselves that whatever it is that’s telling us it’s a good idea to sin willfully is lying; we can ask God to give us repentance to acknowledge the truth (2Ti 2:25-26), choose the fear of God and depart from evil. (Pr 3:7)