When I have preached on Hebrews 12 in the past, I have taken the “root of bitterness” that “grows up and defiles many” to refer to personal bitterness harbored toward another for real or imagined ills that have been done. I have warned people against harboring bitterness and urged them to forgive those who have wronged them.
If a person misses the grace of God (I have said), “there will be a price to pay – or, to be more precise,there will be hell to pay: a bitter root will grow up, a root that springs from the very soil of hell; and it will ’cause trouble and defile many.’”
Many years of pastoral experience have led me to this conclusion, and I believe it is an accurate one. I have seen people’s lives, marriages, relations to children and parents, and mental health destroyed because they harbored bitterness and refused to forgive.
I believe this warning remains true. I further believe it has biblical support. I have begun to doubt, however, that this is the point Hebrews 12:15 is making.
This past week, I was reading Deuteronomy 28-30, one of the foundational passages for coming to grips with how Jews in the post-exilic period understood their situation, when I came upon Deuteronomy 29:18. The NIV translates: “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.”
I noticed the similarities with Hebrews 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.” Knowing that the author of Hebrews routinely quotes from the Septuagint (the Greek-translation of the Old Testament, completed prior to Christ), I looked up the Deuteronomy passage in the LXX (the Septuagint). Greek readers can find the passage below.
Deut. 29:18 (17 in LXX) μή τίς ἐστιν ἐν ὑμῖν ἀνὴρ ἢ γυνὴ ἢ πατριὰ ἢ φυλή, τίνος ἡ διάνοια ἐξέκλινεν ἀπὸ κυρίου τοῦ θεοῦ ὑμῶν πορεύεσθαι λατρεύειν τοῖς θεοῖς τῶν ἐθνῶν ἐκείνων; μή τίς ἐστιν ἐν ὑμῖν ῥίζα ἄνω φύουσα ἐν χολῇ καὶ πικρίᾳ; 
Now here is Hebrews 12:15 in Greek: ἐπισκοποῦντες μή τις ὑστερῶν ἀπὸ τῆς χάριτος τοῦ θεοῦ, μή τις ῥίζα πικρίας ἄνω φύουσα ἐνοχλῇ καὶ διʼ αὐτῆς μιανθῶσιν πολλοί,
Note the repetition of 8 words from Deuteronomy. Also note the similar sounding ἐνοχλῇ (Hebrews) and ἐν χολῇ (Deuteronomy). I have no doubt that the author of Hebrews was thinking of Deuteronomy when he wrote.
That may give a different meaning to what the author of Hebrews wrote than what I have understood in the past. Falling short of the grace of God may be in his mind parallel to ἐξέκλινεν ἀπὸ κυρίου τοῦ θεοῦ ὑμῶν (“turning away from the Lord your God”) to the worship/service of the gods of the nations. This seems especially fitting when we remember the warnings against drifting away, turning away (3x), and falling away made elsewhere by the author of Hebrews.
If this is so, the root of bitterness is not the bitterness I feel toward another person but the bitterness I suffer (as in Deuteronomy) for missing the grace of God by turning away from him and to other things. It is a bitterness that spreads, affecting not only me but the people around me.
I am sure than many people have seen and commented on this. Somehow I had missed it until now. There are always new and interesting (and often challenging) things to discover in the Scriptures.
 Nestle, E., Nestle, E., Aland, B., Aland, K., Karavidopoulos, J., Martini, C. M., & Metzger, B. M. (1993). The Greek New Testament (27th ed., Heb 12:15). Stuttgart: Deutsche Bibelgesellschaft.
 Septuaginta: With morphology. (1996). (Dt 29:17). Stuttgart: Deutsche Bibelgesellschaft.