Pursue peace with all people, and holiness, without which no one will see the Lord: 15looking carefully lest anyone fall short of the grace of God; lest any root of bitterness springing up cause trouble, and by this many become defiled; 16lest there be any fornicator or profane person like Esau, who for one morsel of food sold his birthright. 17For you know that afterward, when he wanted to inherit the blessing, he was rejected, for he found no place for repentance, though he sought it diligently with tears.
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.
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. [SOURCE: Desiring God]