I wish I could write to you at length about grace. It is a fascinating subject. You are quite right in calling it “a participation of the divine nature,” since Scripture uses almost the same words to describe it. A comparison of the Fathers of the Church helps to explain things a little. A piece of iron, they say, placed in a fire does not in reality change its nature, yet it seems to do so; it burns and glows like the fire around it, it cannot be distinguished from the fire. In similar wise a soul clad in grace borrows beauty and magnificence from God’s beauty and magnificence; it seems to partake of the nature of God. What joy to remember that every tiny thing done for God, an act, a word, a glance even, brings fresh grace to the soul, makes it partake more and more of the nature of God, until St. Paul has to exclaim: “I have said you are gods!” and no longer mortals. Our Lord longs for this transformation, and so He sends many hard trials to hasten the day of this perfect union. Let Him, then, have His way. You can have perfect confidence that He is doing the right thing ever and always. Holiness is really nothing more than perfect conformity to God’s Will, and so every step in this direction must please Him immensely.
However, this transformation can also be physical in some way in the lives of the saints. There is a temptation to discount such phenomena as part of as mythical “Golden Legend” of the saints. Sometimes it can be good to be a little sceptical about mystical phenomena, but it is surely not the Christian position to completely and automatically dismiss such phemonena out of hand entirely.
We read in the lives of many saints about how, on occasion, others thought that they could perceive a certain radiance around them. The Book of Exodus tells us how the face of Moses was shining and radiant after he came down from the presence of the Lord on Mount Sinai. These tales are not confined to the distant past; for instance there have been reports of how acquaintances of St John Paul II perceived that his face also shone on occasion. Those who were present at the apparitions at Lourdes also reported a radiant look on the face of St Bernadette during her visions, and it was the power of this radiance that convinced them of the authenticity of the visions. Similarly, those present when St Pio said Mass could also perceive a radiance in his face.