'Be Careful' in the Bible

Be carefully on your guard lest there be any one who falls back from the grace of God; lest any root bearing bitter fruit spring up and cause trouble among you, and through it the whole brotherhood be defiled;

Be careful not to refuse to listen to Him who is speaking to you. For if they of old did not escape unpunished when they refused to listen to him who spoke on earth, much less shall we escape who turn a deaf ear to Him who now speaks from Heaven.