Matthew Poole and continuators on Hebrews 3:12

12 Take heed Brethren , lest there be in any of you an evil heart of unbelief in departing from the living God .

Here the Spirit applieth the former dreadful example of Sin and Judgment to the Hebrews, to forewarn them how they sinned as these did, lest they partake of the like Vengeance, and so enters his Caution against Unbelief. [non-Latin text not transcribed] signifies not an act of Sight, but of the Mind, Circumspection, Watchfulness, and Heed, taking exactest caution of the Evil forbidden, chap. 12.15, 25. 1 Cor. 8.9. Brethren they were to Paul in the Flesh, and more so as True Believers in Christ, he cautions them particularly, one by one, lest any Root of Bitterness should be amongst them, chap. 12.15.

The Heart is the first, and proper, and chiefest subject wherein all Sin riseth, and from thence issueth into Words and Works, Matth. 15.18, 19. compare Iames 1.14, 15. This comprehendeth the Mind, Will and Affections, the whole inward Man; and this Heart in every Man is naturally and habitually Evil, continually forging and framing of it, Gen. 6.5. compare Ierem. 17.9. Almighty Grace only can change this Heart, yet it works by Counsel, and makes the Soul willing to use the means appointed to effect it. VNBELIEF, though but in it self, is but a denial to assent to, or rely on the Will of God revealed to it, yet is the Spring and Fountain of all other Sin, the teeming Womb from whence all issueth, as, Uncleanness, Idolatry, Unrighteousness, Superstition, &c. It was the hardening Sin of their fore-fathers, they would not Believe, and then did Murmur and Rebel: It is the root of Apostacy; Men breaking their Covenant with God in Christ, do then desert him: Against this Persidious, Impious, perverted Temper doth he caution them to watch, that neither for measure nor season they ever do admit or allow it; that there be not at all in the least degree, or at any time such a base, malignant quality in their Hearts, Rom. 11.20, 21.

Turning away, standing off, and separating the Heart, it implies in it a real, total, final defection; actual and formal Apostacy from him whom they had owned and received, and is actual Rebellion against their lawful Soveraign, by turning either Jews or Heathens, and renouncing the Christian Religion and its Author, who is the Living God, not only formally as opposed to dead Idols, but efficiently the Author and Fountain of all sorts of Life, but especially of Spiritual and Eternal Life, Iohn 5.19, 20, 21, 25, 26. which Living God is our Lord Jesus Christ, ver. 7. whose Voice they were to hear, who was tempted by their Unbelief in the Wilderness, 1 Cor. 10.9. who gave the Law to them at Sinai, chap. 12.26. So that to apostatize from him and his Religion, is to apostatize from God, and to renounce Eternal Life, and to subject themselves to Eternal Punishments, which he ever liveth to inflict on them; Unless they took heed to avoid this Unbelief, it was impossible for them to persevere in Christianity, when threatned with Persecutions, and the loss of Peace, Liberty, Safety, Estates, Honours, Relations, and Life it self for it.