What is "grace?" It is God's own life, shared with us. God's life is Love. Deus caritas est. By grace we are able to share in the infinitely selfless love of [God] Who is such pure actuality that [God] needs nothing and therefore cannot conceivably exploit anything for selfish ends. Indeed, outside of [God] there is nothing, and whatever exists exists by [God's] 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 on [God's] gift, depends upon [God's] 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 [human], 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 and above 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.
So the natural goodness of [human], [the human] capacity for love which must always be in some sense selfish if it remains in the natural order, becomes transfigured and transformed when the Love of God shines in it.
Thomas Merton, The Seven Storey Mountain