The sun shining down on wax will melt the wax. The same sun shining down on a lump of clay will harden the clay. It all depends on the properties that make up the wax and the clay. A person's hardness or softness of heart will depend on how that individual responds to the light of the word of God. We can either respond or reject and our hearts corespondingly, will either be softened or hardened. It all depends on whether we will hear and receive the truth of the word of God or not. It is infinitely better to fall on the Rock, Christ Jesus, than to have the Rock fall on you.