So, as the Holy Spirit says: “Today if you hear his voice, do not harden your hearts as you did in the rebellion, during the time of testing in the desert, where your fathers tested and tried me and for forty years saw what I did.  That is why I was angry with that generation, and I said, “Their hearts are always going astray, and they have not known my ways.”  So, I declared on oath in my anger, “They shall never enter my rest.”

Warning against unbelief - Hebrews 3:7-14

See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God.  But encourage one another daily. as long as it is called Today, so that none of you may be hardened by sin’s deceitfulness .  We have come to share in Christ if we hold firmly till the end the confidence we had at first.