Root of bitterness

Watch that no root of bitterness grow up, Hebrews 12:15 warns. What’s a root of bitterness? Sounds like someone who’s bitter in the church. But Deuteronomy 29:18 uses a similar phrase to describe those who turn aside from Yahweh to serve other gods. This fits the context of Hebrews very neatly: The root of bitterness in Hebrews is the Christian who intends to turn back from Jesus to Moses. With the coming of the New Covenant, that’s an equivalent of a return to Egypt, a return to slavery.