The statement proceeds from the ancient philosophical argument that God is the direct cause of everything that happens on earth. There’s a flood in Kansas? God directly willed it. A baby dies in LA? God literally made it happen. According to that view, everything must be directly ascribed to God’s action. There’s a problem there, though. What about when people choose to act against God’s will? Doesn’t that disprove this philosophical argument? After all, if God makes all things happen, and if I’m able to do something that God doesn’t want, then doesn’t that mean that I’m more powerful than God? In this context, then, we can understand what “hardening of hearts” means. It’s a way of shoring up this ancient philosophy: it’s not that you chose to disobey God… it’s just that God hardened your heart so that you’d act that way. In other words, it’s both the case that you acted poorly and that God’s still in ultimate and direct control. Thank goodness we no longer hold to that philosophical argument, though – if we did, then God would truly be seen an angry puppet master, blessing and crushing us arbitrarily at His whim!
John is pulling from that tradition, though, by quoting Isaiah. How else might we explain why people were able to see Jesus and yet not believe? In more contemporary terms, we might say that God allowed people to turn away from him; that is, we might say that God allows people to harden their hearts against Him.