How Can You Harden Your Hearts So?

There's a time then when it says the people did harden their hearts. We know from Nephi's definition of hardening the heart. He spoke to Laman and Lemiel who did not inquire for themselves about their father's vision of the tree of life. How often do we delve into the scriptures to confirm a thing that we've been led to believe is true or not? Because the scriptures cover it all. If it's out of harmony with the scriptures in any situation, then it's not of God.