12. Take heed--to be joined with "wherefore," Heb 3:7.
      lest there be--Greek (indicative), "lest there shall be"; lest there be, as I fear there is; implying that it is not merely a possible contingency, but that there is ground for thinking it will be so.
      in any--"in any one of you." Not merely ought all in general be on their guard, but they ought to be so concerned for the safety of each one member, as not to suffer any one to perish through their negligence [CALVIN].
      heart--The heart is not to be trusted. Compare Heb 3:10, "They do always err in their heart."
      unbelief--faithlessness. Christ is faithful; therefore, saith Paul to the Hebrews, we ought not to be faithless as our fathers were under Moses.
      departing--apostatizing. The opposite of "come unto" Him (Heb 4:16). God punishes such apostates in kind. He departs from them--the worst of woes.
      the living God--real: the distinctive characteristic of the God of Israel, not like the lifeless gods of the heathen; therefore One whose threats are awful realities. To apostatize from Christ is to apostatize from the living God (Heb 2:3).

JFB.