"What is "grace"? It is God's own life, shared by us. God's life is Love. Deus caritas est. By grace we are able to share in the infinitely selfless love of Him Who is such pure actuality that He needs nothing and therefore cannot conceivably exploit anything for selfish ends. Indeed outside of Him there is nothing and whatever exists exists by His free gift of its being, so that one of the notions that is absolutely contradictory to the perfection of God is selfishness. It is metaphysically impossible for God to be selfish, because the existence of everything that is depends upon His gift, depends upon His unselfishness.
When a ray of light strikes a crystal, it gives a new quality to the crystal.
And when God's infinitely disinterested love plays upon a human soul, the same kind of thing takes place. And that is the life called sanctifying grace.
The soul of man, left to its own natural level, is a potentially lucid crystal left in darkness. It is perfect in its own nature, but it lacks something that it can only receive from outside itself. But when the light shines in it, it becomes in a manner transformed into light and seems to lose its nature in the splendor of a higher nature, the nature of the light that is in it," – Thomas Merton, The Seven Storey Mountain.