“Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.”
King James Version (KJV)
3:12 Take heed, lest there be in any of you - As there was in them. An evil heart of unbelief - Unbelief is the parent of all evil, and the very essence of unbelief lies in departing from God, as the living God - The fountain of all our life, holiness, happiness.
Heb 3:12 Take heed, brethren, etc. They fell from unbelief, which led them to depart from God. Take heed, lest you Hebrew Christians seeking the heavenly rest should so fall.