From “The Seven Storey Mountain” by Thomas Merton.
There is a paradox that lies in the very heart of human existence. It must be apprehended before any lasting happiness is possible in the soul of a man. The paradox is this: man’s nature, by itself, can do little or nothing to settle his most important problems. If we follow nothing but our natures, our own philosophies, our own level of ethics, we will end up in hell.
This would be a depressing thought if it were not purely abstract. Because in the concrete order of things God gave man a nature that was ordered to a supernatural life. He created man with a soul that was made not to bring itself to perfection in its own order, but to be perfected by him in an order infinitely beyond the reach of human powers. We were never destined to lead purely natural lives, and therefore we were never destined in God's plan for a purely natural beatitude. Our nature, which is a free gift of God, was given to us to be perfected and enhanced by another free gift that is not due it.
This free gift is “sanctifying grace." It perfects our nature with the gift of a life, an intellection, a love, a mode of existence infinitely above its own level. If a man were to arrive even at the abstract pinnacle of natural perfection, God’s work would not even be half done: it would be only about to begin, for the real work is the work of grace and the infused virtues and the gifts of the Holy Ghost.
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 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 splendour of a higher nature, the nature of the light that is in it.