What is sin?

"Sin is the refusal of spiritual life, the rejection of the inner order and peace that come from our union with the divine will. In a word, sin is the refusal of God's will and of his love. It is not only a refusal to 'do' this or that thing willed by God, or a determination to do what he forbids. It is more radically a refusal to be what we are, a rejection of our mysterious, contingent, spiritual reality hidden in the very mystery of God. Sin is our refusal to be what we were created to be-- sons of God, images of God. Ultimately sin, while seeming to be an assertion of freedom, is a flight from the freedom and the responsibility of divine sonship."

from Life and Holiness by Thomas Merton