The author asks his readers to leave the elementary teaching about Christ and go on to maturity, not laying again a foundation of repentance from dead works, faith toward God, instruction about washings and the laying on of hands, the resurrection of the dead and eternal judgment. This they will do if God permits.
Then comes the warning. It is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, shared in the Holy Spirit and tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, since they crucify the Son of God again for themselves and put him to open shame. The author illustrates it with land that drinks the rain and bears a useful crop, receiving a blessing from God, while land that bears thorns and thistles is worthless, near to being cursed, and ends up burned.
He immediately softens the tone: though we speak this way, beloved, we are sure of better things in your case, things that belong to salvation. God is not unjust so as to overlook the work and love they showed in serving the saints. He wants each to show the same diligence to the end, for the full assurance of hope, so that they may not be sluggish but imitators of those who through faith and patience inherit the promises.
The second half of the chapter rests on Abraham. When God made his promise, since he had no one greater to swear by, he swore by himself, and Abraham, having patiently waited, obtained the promise. Through two unchangeable things, the promise and the oath, in which it is impossible for God to lie, we who have fled for refuge have strong encouragement to hold fast the hope. This hope is a sure and steadfast anchor of the soul that enters behind the curtain, where Jesus has gone as a forerunner on our behalf, having become a high priest forever after the order of Melchizedek.