Many people speak about illumination these days, but not all understand what it really is. Illumination is the illumination of the heart of our souls, the nous. When the nous is illuminated through divine grace – what the Eastern Christians, especially the desert masters, continually strived for (and some attained) through the combination of hesychia and grace – it becomes both oriented toward and occupied with God alone.

However, this does not mean that after illumination, we will forget everything and just want to become monks. What occurs if we accept, rather than reject, the grace itself of illumination, is that our thoughts – thoughts not concerned with God – will become expelled from the very heart of our souls, and the Presence of God the Father, through the Holy Spirit, will descend to live in the heart of our souls – the spiritual coming of the Kingdom. Making us capable of being engaged in earthly tasks while concurrently worshipping Our Father from our hearts in Spirit and in Truth: prayer of the heart.

The aforementioned becomes possible because it entails union with God. This is the sheer beauty of the illumination of the nous, the heart of our souls, which is forthcoming. It is precisely the grace that has been given to various saints over the ages and now it is going to be given to everyone, to accept or reject according to their own free will. That is the Majesty of Our Father, that is the beauty of His forthcoming, unparalleled, outpouring of grace upon the world and all mankind.

Reference

Letter of Basil the Great to Gregory Nazianzen.