“Bitterness” is usually associated with anger and grudges. But is this what it means in Hebrews 12:15: “See to it that no one fails to obtain the grace of God; that no ‘root of bitterness’ springs up and causes trouble, and by it many become defiled”? I don’t think so.
Let’s ask a few questions. First of all, does “root of bitterness” mean that the root is bitterness (like block of wood)? Or does it mean that the root grows up into a plant and bears the bitter fruit? Second, does “bitterness” in Hebrews 12:15 mean “festering anger,” or does it mean “poisonous and foul”? Third, where did this image of a “root of bitterness” come from?
Let’s start with the last question. Answer: it came from Deuteronomy 29:18.
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.
This background also helps us answer the first two questions: the root is not itself bitterness but rather bears the fruit of bitterness. And the bitterness it bears is something poisonous. This bitter fruit may be festering anger, or it may be something else. The point seems to be that it is deadly.
The key question is this: What is this root that causes deadly, bitter fruit to sprout in the church? The next verse in Deuteronomy 29 gives the surprising answer, but it fits perfectly with the book of Hebrews. Deuteronomy 29:18 ends, “Beware lest there be among you a root bearing poisonous and bitter fruit.” Then verse 19 begins by defining this root:
. . . 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.
What then is the root that brings forth the bitter fruit? It is a person who has a wrong view of eternal security. He feels secure when he is not secure. He says, “I shall be safe [secure], though I walk in the stubbornness of my heart.” He misunderstands the covenant God makes. He thinks that because he is part of the covenant people, he is secure from God’s judgment.
This kind of presumption is what the book of Hebrews deals with repeatedly — professing Christians who think they are secure because of some past spiritual experience or some present association with Christian people. The aim of Hebrews is to cure Christians of presumption, and to cultivate earnest perseverance in faith and holiness. At least four times it warns that we must not neglect our great salvation but be vigilant to fight the fight of faith every day lest we become hardened and fall away and prove that we had no share in Christ (Hebrews 2:3; 3:12–14; 6:4–7; 10:23–29).
This is also the very point of the context of the term “root of bitterness” in Hebrews 12:15.
Strive for peace with everyone, and for the holiness without which no one will see the Lord. See to it that no one fails to obtain the grace of God; that no “root of bitterness” springs up and causes trouble, and by it many become defiled. (12:14–15)
This is a warning not to treat holiness lightly or to presume upon more grace.
Therefore a “root of bitterness” is a person or a doctrine in the church which encourages people to act presumptuously and treats salvation as an automatic thing that does not require a life of vigilance in the fight of faith and the pursuit of holiness. Such a person or a doctrine defiles many and can lead to the experience of Esau who played fast and loose with his inheritance and could not repent in the end and find life.