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. (NIV)
This verse, I believe, has been mangled by many, including John Piper, over the years. I heard his take on this verse in the early 1990s somewhere and was disappointed in his exegesis then as well as now. He is still holding to his view as of 2018: https://www.desiringgod.org/messages/an-impossible-covenant/excerpts/church-wont-save-you
Piper sees the exhortation as warning of the false security of belonging to a group. This idea is found in vs. 19: When such a person hears the words of this oath and they invoke a blessing on themselves, thinking, “I will be safe, even though I persist in going my own way,” they will bring disaster on the watered land as well as the dry. The text seems fairly clear that “going my own way” equates with following other gods of vs. 18.
Piper’s idea, however, says that empty membership is the problem. While false confidence is offensive, it is the result, but not the underlying offense. The text is clear that the offense is idolatry and it is hidden (roots are buried and not seen – the person is living a double life). The root is poisonous – no life can spring from it. The other gods of the nations do not give life is what the text seems to be saying when it speaks of “poisonous.” The self – confidence is empty, yes, but, at the core, they are captivated by other gods, is the real problem.
Piper’s reading of Heb. 12.15, which quotes Dt. 29.18, is also problematic. How he attributes “holiness” of the previous verse as the solution to the quote is mystifying. A better connection would be Esau or the immoral person of the following verses. A case can also be made to see the “missing the grace of God” as someone who has returned to their former Jewish observances secretly perhaps, but also wants to worship Christ, possibly, to hedge their bets. Therefore, this person would need to live a double life.
Regardless, it seems the hidden root of Heb. 12.15 will be judged by God and will affect others related to it. This is what Dt. 29.19 seems to allude to as well with the reference to “the watered and dry” [land].