5 Our whole business then, Brethren, in this life is to heal this eye of the heart whereby God may be seen. To this end are celebrated the Holy Mysteries; to this end is preached the word of God; to this end are the moral exhortations of the Church, those, that is, that relate to the correction of manners, to the amendment of carnal lusts, to the renouncing the world, not in word only, but in a change of life: to this end is directed the whole aim of the Divine and Holy Scriptures, that that inner man may be purged of that which hinders us from the sight of God.
For as the eye which is formed to see this temporal light, a light though heavenly, yet corporeal, and manifest, not to men only, but even to the meanest animals (for for this the eye is formed, to see this light); if anything be thrown or fall into it, whereby it is disordered, is shut out from this light; and though it encompass the eye with its presence, yet the eye turns itself away from, and is absent from it; and through its disordered condition is not only rendered absent from the light which is present, but the light to see which it was formed, is even painful to it. So the eye of the heart too when it is disordered and wounded turns away from the light of righteousness, and dares not and cannot contemplate it.
Source: Sermons on Selected Lessons of the New Testament (New Advent)