V.23 We are told to hold fast, and then we are reminded that, although we are weak, yet He is always faithful.
V.26-31 In spite of that one sacrifice and all its perfect value, there still was the danger that some might profess to be Christians, and later turn away entirely. For such, there was no hope, nothing left but punishment. These were never believers. Those who do not believe in eternal security, use these along with Hebrews 6:46 and 2 Peter 2:20-22 to try to prove that a person could be lost after having once been saved. (But see John 10:28).
V.29 “Sanctified.” If a person turned from Jewish sacrifices, and took his place among Christians by profession, he was in that way set apart from those who were still religious Jews. He has the name of a “Christian.” If such a professor turns against the Christianity which he professed, see what words are spoken to such a man.
V.32-34 The marks of real believers in Christ.
V.39 There were many whose profession of faith in Christ was real.