12:12-15 What is the teaching here?

Christians are being exhorted here to reinvigorate themselves spiritually and walk in the way God has ordained for them to be partakers of His holiness (CP V9-11 with Pr 4:25-27; Eph 1:4).

Strong Christians must never exhibit any spiritual slackness so that weaker Christians may be encouraged and strengthened in their faith (CP Isa 35:3-4). Any root of bitterness in He 12:15 refers metaphorically to any Christian apostatising and failing God's grace, and inducing others to do the same (CP De 29:18; He 3:12-13). Many in the church believe that He 12:15 is about bitterness, but it is about apostasy.

There are also many in the church who believe that the book of Hebrews is directed to non-Christians, but that is not correct either. It is directed to Christians, as these passages clearly prove. Only Christians can lay aside every weight and the sin that so easily besets them, and only Christians can run with patience the Christian race (CP He 12:1).

Only Christians can consider Jesus, and look to Him as the originator of faith and the one who raised it to perfection (CP V2-4). Only Christians can recognise and submit to God's chastening as true sons in order to partake of His holiness (CP V5-11). And only Christians can follow peace and holiness and meet other terms of blessing lest they fail God's grace (CP V12-15).