The Bible talks about a hardened heart like an untended and dry field, when people turn from the truth of God's word their heart/soul becomes dry and barren and hardened against the truth. I found this in my notes;
For thus saith the LORD to the men of Judah and Jerusalem, Break up your fallow ground, and sow not among thorns.
Sow to yourselves in righteousness, reap in mercy; break up your fallow ground: for it is time to seek the LORD, till he come and rain righteousness upon you.
I heard a great sermon from this scripture one time. I can only remember bits and pieces of it but I remember the point of the message. Fallow ground was described as land that had been left untended and dry for years. The soil had become hard packed and dead because there was no oxygen or moisture in it. The surface had become almost as hard as rock and any rain just ran right off of it without soaking in.
In order to tend fallow ground you needed a special plow that was very heavy and strong. Then you needed a large team of Oxen in order to pull the plow and break the ground. You had to start out plowing shallow and then with each successive pass you increase the depth until the ground is sufficiently broken.
When a soul has been hardened through lack of tending and drought, then it takes this kind of plowing to soften it up for use in the Lord's vineyards. In other words it takes a heavy dose of the word of God by Christians strong in faith. They must first start out with the simple truths and then successively get deeper into the Word of God as the heart begins to soften. No doubt it takes prayer, fasting, faith and diligence in order to soften such a heart.