"Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts." (Hebrews 4: 7)
In this verse, the writer of Hebrews quotes from Psalm 95, in which the Psalmist pleads with the Israelites to believe God, that He has prepared a full and ready Promised Land for them.
In the first movement toward the Promised Land, they were overwhelmed with unbelief:
"25And they returned from searching of the land after forty days. 26And they went and came to Moses, and to Aaron, and to all the congregation of the children of Israel, unto the wilderness of Paran, to Kadesh; and brought back word unto them, and unto all the congregation, and shewed them the fruit of the land. 27And they told him, and said, We came unto the land whither thou sentest us, and surely it floweth with milk and honey; and this is the fruit of it. 28Nevertheless the people be strong that dwell in the land, and the cities are walled, and very great: and moreover we saw the children of Anak there. 29The Amalekites dwell in the land of the south: and the Hittites, and the Jebusites, and the Amorites, dwell in the mountains: and the Canaanites dwell by the sea, and by the coast of Jordan." (Numbers 12: 25-29)
They were looking at themselves, not at the LORD who had brought them from Egypt to the border of the Promised Land.
Their hearts were hardened, unwilling to listen to the words of Moses or the two spies who gave a good report:
"And Caleb stilled the people before Moses, and said, Let us go up at once, and possess it; for we are well able to overcome it." (Numbers 13: 30)
Instead, wandering in unbelief, the Israelites mourned:
"1And all the congregation lifted up their voice, and cried; and the people wept that night. 2And all the children of Israel murmured against Moses and against Aaron: and the whole congregation said unto them, Would God that we had died in the land of Egypt! or would God we had died in this wilderness! 3And wherefore hath the LORD brought us unto this land, to fall by the sword, that our wives and our children should be a prey? were it not better for us to return into Egypt? 4And they said one to another, Let us make a captain, and let us return into Egypt." (Numbers 14: 1-4)
Whether we like it or not, unbelief is the norm for human beings. In fact, God understands this, and has provided a way out of it:
"And I will give them one heart, and I will put a new spirit within you; and I will take the stony heart out of their flesh, and will give them an heart of flesh." (Ezekiel 11: 19)
Notice that part of New Covenant includes a new heart. The English translation does not bring out the fullness of what this new heart is and does. Another translation:
"And I will give them singleness of heart and put a new spirit within them. I will take away their stony, stubborn heart and give them a tender, responsive heart," (Living Translation)
A tender, soft heart. Also, the word for flesh "Bashar" also means "Good News", or the Gospel of Grace.
When we believe, accept the Gospel of Grace (Galatians 1: 6), that we have been justified from all things through Jesus (Acts 13: 38-39), God is able to give us a soft heart, and we will believe all that He says.