We should avoid a hard heart, an unbelieving heart…

Afterward He appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen Him after He was risen.

Mark 16:14 KJV

Today, if you hear his voice,
do not harden your hearts
as you did at Meribah,
as you did that day at Massah in the desert,
where your fathers tested and tried me,
though they had seen what I did.

For forty years I was angry with that generation;
I said, "They are a people whose hearts
go astray, and they have not known my ways."

Psalm 95:7-10 NIV

Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. But encourage one another day after day, as long as it is still called ‘Today,’ so that none of you will be hardened by the deceitfulness of sin. For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end.

Hebrews 3:12-14 NASB

And He (Jesus) said to him,

"You shall love the Lord your God
with all your heart,
and with all your soul,
and with all your mind.

This is the great and first commandment.

Matthew 22:37,38 RSV