John Calvin Commentary Hebrews 3:12

"Take heed, brethren, lest haply there shall be in any one of you an evil heart of unbelief, in falling away from the living God:" — Hebrews 3:12 (ASV)

Take heed, (or See,) brothers, lest there be at any time in any of you a wicked heart of unbelief, etc. I have preferred to retain literally what the Apostle states, rather than to give a paraphrase regarding the wicked or depraved heart of unbelief, by which he implies that unbelief would be connected with depravity or wickedness, if after having received the knowledge of Christ they departed from his faith. For he addressed those who had been imbued with the elements of Christianity; therefore, he immediately added, By departing; for the sin of defection is accompanied by perfidy.