This verse focuses on bitterness. What is it? Sinful worship, relationships and attitudes such as resentment, grumbling, complaining are all included as poisonous fruit of this invasive and malignant weed. For those sins God judged His wilderness people, and warns His New Testament church (1 Corinthians 10:6-10). But those sins are only the obvious fruit of bitterness, not the core of the problem.

The definition of bitterness is found in Deuteronomy 29:18-19. This was the occasion when Moses exposed the cancer of the bitter heart when he explained God's law to the Israelites a second time, before they were due to enter the Promised Land: "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. 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 fundamental problem is a self-righteous arrogance in rejecting God's way, presuming eternal safety despite going their own way, refusing to repent and assuming that God's judgement cannot touch their sin. A heart that beats like that will be a dangerous example within the church as it is the exact opposite of how God wants His people to be. Perverted worship, and relationships go together with God-despising conceit. It is a highly contagious spiritual disease, and unless it is rooted out, it will make the church dysfunctional, leading people astray and failing to make disciples; and the perpetrators are at risk of losing everything.

It is a perennial problem for the church. Either we depend on the grace of Christ or we depend on ourselves. So, those who believe they can please God through their own efforts are already bitter in their hearts against Him; they despise being weak and needing to rely on the Lord. They will teach a legalistic religion devoid of grace, which cannot sanctify their followers. Those who are infected will resist the Lordship of Christ, preferring their own Lordship and have nothing to offer a broken world. The root which causes bitterness may lie deeply hidden beneath a pile of good deeds and traditional orthodoxy, but their hearts are not utterly dependent on the grace of Christ. As the writer says, we must 'see to it'; examine ourselves and repent before more damage is done, and the Lord proclaims that He does not know us (Matthew 7:21-23).