Perhaps, it’s a too generalized statement that most folks misunderstand Heb.12.15, but, when even pastors use the phrase to refer to those with bitter spirits, the phrase misleads. It makes it sound like bitter folks are sinning, which is far from the truth. To equate the biblical concept “root of bitterness” to those bitter of heart misses the mark.

Perhaps it is more a matter of rendering Koine Greek into English. If it was rendered “poisonous root” then it better conveys the meaning of the concept.

Heb.12.14-17 gives instruction to holiness and the example of Esau as one to avoid being completely profane and without spiritual understanding. This doesn’t align well, or at all, when speaking of those bitter with life, just the opposite. The “root of bitterness” in vs. 15 is not defined and is a call for the Jewish readers to examine the concept in Dt. 29. There, as well, we find an arrogant, profane person resembling the author’s use of Esau in Hebrews.

Dt. 29 records Moses cutting the Moab Covenant. The Israelites are going to swear to it. This shows another covenant subsequent to The Sinai Covenant and suggests many things, not the least being the New Covenant promised in the future by Jeremiah (31.31-34). Also, this sworn agreement on the Plains of Moab resembles the grace of the New Covenant, since it is a matter of a sworn oath for them to be loyal to the God of Israel, exclusively. The Sinai Covenant gave an impossible standard for fallen humans, while the Moab Covenant gave them an inheritance forever with God. Only One Person could fulfill the Sinai Covenant and reap the reward of life for His followers (see Gal. 3.19b).

The Moab Covenant was gracious in that it didn’t look at the performance of the children. They would now be family and loved in their imperfection. God was going to accept them despite their falleness if they were true in their heart to Him.

So the writer of Hebrews chooses an important historical marker in Dt. 29 which would have resonated in the minds of his readers. The person producing evil fruit occupies the main part (vss.16-28) of the whole account, detailing the woes that are the consequence of disloyalty.

Dt. 29.18-19 refers specifically to the term in Hebrews 12.15 and illuminates the concept. It is not speaking of bitterness of soul, but of arrogance, an opposite concept altogether. Here is the section in English: Beware lest there be among you a man or woman or clan or tribe whose heart is turning away today from the LORD our God to go and serve the gods of those nations. Beware lest there be among you a root bearing poisonous and bitter fruit, one who, when he hears the words of this sworn covenant, blesses himself in his heart, saying, ‘I shall be safe, though I walk in the stubbornness of my heart.’ This will lead to the sweeping away of moist and dry alike (ESV).

Bitterness (poison) is the outcome of worshipping other gods on the sly. “The root” is metaphorical for what the person is in their heart: “say to themselves.” This is the inner life of the person. They are not a plant above ground producing good fruit; instead, they are a mere ornamental plant (one not producing fruit), that hides a poisonous secret in their heart. They bless themselves thinking they can break their oath (sworn covenant), exhibiting that they know better than the God who saved them from Egypt.

Rather than to think of individual “bitterness of heart” as sinful, it may instead just be the consequence of a fallen world. I see nowhere in scripture where “bitterness of soul” is condemned. While God’s people should rejoice always, yet, in this life, each heart knows its own bitterness. 1Sam. 22.1-2 gives the account of David’s followers to whom he gave refuge and led when fleeing from both Saul and the Philistines. Some of these were “bitter of soul” about their situation: David departed from there and escaped to the cave of Adullam. And when his brothers and all his father’s house heard it, they went down there to him. And everyone who was in distress, and everyone who was in debt, and everyone who was bitter in soul, gathered to him. And he became commander over them. And there were with him about four hundred men (ESV).

Back in Dt. 29.19, it mentions the sweeping away of the moist and dry as a consequence of this sin of internal deceit and arrogancy. This may refer to his family members or tribe whom he has influenced (the dry) and those who themselves were innocent (the moist) of this particular sin. This aligns well with the text in Hebrews 12.15 where the presence of the evil growth “springs up and causes trouble, and by it many become defiled.”

In Luke 23.31, Jesus uses the metaphor of dry and green (moist) to describe the age during His time. The green time was during the ministry of John the Baptist and Jesus: it was the blessing of God in sending the Messiah and His ministries of teaching and healing to the people. Rhetorically, Jesus asks what would evil men do during spiritual dryness. He would later commission Paul to describe that time: But understand this, that in the last days there will come times of difficulty. For people will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, heartless, unappeasable, slanderous, without self-control, brutal, not loving good, treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, having the appearance of godliness, but denying its power. Avoid such people. (2Tim.3.1-5 ESV).