Hebrews 3:12-14 (ESV)
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.  But exhort one another every day, as long as it is called "today," that none of you may be hardened by the deceitfulness of sin.  For we share in Christ, if indeed we hold our original confidence firm to the end.
Who doesn’t have an unbelieving heart? I know I have an unbelieving heart. It is my heart that leads me to sin. Every sin is a sign of unbelief. Every sin shows that I do not fear, love, and trust in God above all things. Our hearts are evil. It is that simple. Jesus says so. “Out of the heart come…” So every day, as long as it is called today, is a day in which we are called to repentance, to drown our old Adam in our baptisms, to rejuvenate our faith. This is why God gave us his word, and communion. A Christian's devotional life is the carrying out of daily repentance. Repentance is much more than saying sorry for sins. To repent is to believe in Christ. At least Christian repentance is. So we exhort one another, not only as warnings against sin, but as encouragement to good works, and all starting with the word of God the only source and norm for faith in Christ, out original confidence, we hold firm to the end.