Today!
Be careful then, dear brothers and sisters. Make sure that your own hearts are not evil and unbelieving, turning you away from the living God. You must warn each other every day, while it is still "today", so that none of you will be deceived by sin and hardened against God.
(Hebrews 3:12-13NLT).

From these two verses we are led to understand that amongst others, unbelief is a sin and it turns us away from God. When we operate in unbelief we are unable to receive what God has for us because whoever comes to God must first believe that He is. He is who he says he is, He can and He will do whatever He says. He is not a man that he should lie nor the son of man that he should repent.
We receive from God according to our faith and without faith it is impossible to please God.
As powerful as Jesus is, he could not perform any miracles in his hometown Nazareth because the people where too familiar with him and could not perceive him as the Messiah.
We also learn from this chapter that sin is deceptive and it hardens our hearts toward God, when a heart is hardened it rebels against God. This is why we are told to guard our hearts with all diligence for out of it flows the issues of life, (Proverbs 4:23 paraphrased).
We are encouraged to warn each other everyday against the deceitfulness of sin, taking inventory of our own hearts. How do we encourage each other? one may ask;
We do so by sharing the word of God
Testifying of God's goodness in our lives
Praying for each other
Biblical fellowship with one another
Attending church - for we go from strength to strength as we appear before God in Zion (Psalm 84:7)
As we meditate on the word of God daily may we become more aware of the deceitfulness of sin, most importantly how powerful the blood of Jesus is and that we are no longer slaves to sin.
We are empowered to live like Christ through the blood of Jesus that was shed on Calvary for us.
In our own strength we cannot overcome evil but thank God through the power of the Holy Spirit we are enabled to live like Christ.
The Bible says my people perish because of lack of knowledge so it is important that we stay rooted in the word of God for that is where we gain our power from.
Prayer
Lord keep us from sin, lead us not into temptation but deliver us from evil.
Holy Spirit help us to believe God, Father we ask that you give us the grace to obey you. May we be a people that live a life that is worthy of the calling you have placed on us. We ask Father that you give us pure hearts, remove any heart of stone and give us hearts that are responsive to you in Jesus name. Amen!