Hebrews 3 contains a strong warning about the consequences of a hard, unbelieving heart. "They shall never enter my rest," says the author, quoting Psalm 95:11.
Strong stuff, especially since the people Psalms is talking about are the Israelites of the Exodus. Despite their miraculous salvation by God's own hand, they repeatedly, continually turned away from Him over a period of decades.
How fickle is the human heart! We so easily forget what God has done for us. It is good that God is merciful and compassionate. As Psalm 130 says, if He kept a record of sins, we would be undone. But the blood of Christ has washed away all our sins, removed them as far as the east is from the west.