The illumination of one’s soul, better known as the illumination of conscience, is in essence and fact the healing of the soul that is ill, for it entails reopening the heart of the soul, the nous, to God the Father and His Truth, Jesus Christ; hence, to Knowledge – true (interior, experiential) knowledge – of God, after it has been closed and repeatedly confirmed in its closedness through systemic, personal sin. We are presently incapable of healing and reopening the heart of our souls, even though we may regularly attend Confession, with sincere contrition in our hearts, for “only the Holy Spirit can purify the nous” (Diadokos of Photiki). That is why, more often than not, we keep on repeatedly relapsing into sin despite all our good intentions, because even though our soul is wiped clean after Confession, through the divine grace of the sacrament, the nous still remains closed.
Therefore, illumination – or rather, the re-illumination – of the nous, through the illumination of conscience, is needed for our souls, so that we can start (1) regaining true knowledge of God, especially our Father, while still being down here on earth; (2) living in union with Him, once we accept the grace of illumination – we are, indeed, free to reject it should we wish to do so, although there are immortal, perilous consequences to such a decision; and (3) start actively participating in His Divine Family life by actually living in Him, not just with Him. Since it is a form of divine grace, illumination – once we accept it – will cause us to significantly decrease, if not outrightly eliminate our willingness to sin, especially deliberately, because, for many of us, we cannot know God – either truly know or start truly knowing God – and want to sin at the same time.