Ephesians 4:18

By "hardness of heart" he means a complete lack of remorse. When parts of the body are hardened, they feel no sensation, as though they were completely dead. This may happen to the heart.

So then, brothers and sisters, after the God of truth I address to you an appeal that you may give heed to the words written, that you may save both yourselves and him who reads an address in your midst. For as a reward I ask of you repentance with the whole heart, while you bestow upon yourselves salvation and life. For by so doing we shall set a mark for all the young who wish to be diligent in godliness and the goodness of God. And let not us, in our folly, feel displeasure and indignation, whenever any one admonishes us and turns us from unrighteousness to righteousness. For there are some wicked deeds which we commit, and know it not, because of the double-mindedness and unbelief present in our breasts, and our understanding is darkened by vain desires. Let us, therefore, work righteousness, that we may be saved to the end. Blessed are they who obey these commandments, even if for a brief space they suffer in this world, and they will gather the imperishable fruit of the resurrection. Let not the godly man, therefore, grieve; if for the present he suffer affliction, blessed is the time that awaits him there; rising up to life again with the fathers he will rejoice for ever without a grief.

There is such a thing as being in the dark, even while the light is shining, when the eyes are weak; and weak they become, either by the influx of ill humors, or by superabundance of rheum. And so surely is it also here; when the strong current of the affairs of this life overwhelms the perceptive power of the understanding, it is thrown into a state of darkness. And in the same way as if we were placed in the depths under water, we should be unable to see the sun through the quantity of water lying, like a sort of barrier, above us, so surely, in the eyes of the understanding also a blindness of the heart takes place, that is, an insensibility, whenever there is no fear to agitate the soul. "There is no fear of God," it saith, "before his eyes"; and again, "The fool hath said in his heart, There is no God." Now blindness arises from no other cause than from want of feeling; this clogs the channel; for whenever the fluids are curdled and collected into one place, the limb becomes dead and void of feeling; and though thou burn it, or cut it, or do what thou wilt with it, still it feels not. So is it also with those persons, when they have once given themselves over to lasciviousness: though thou apply the word to them like fire, or steel, yet noting touches, nothing reaches them; their limb is utterly dead. And unless thou canst remove the insensibility, so as to touch the healthy members, everything thou doest is vain.