Merton on Sin

From Life and Holiness:

As St. Paul reminds us (1 Cor. 6: 19), we are "not our own." We belong entirely to Christ. His Spirit has taken possession of us at baptism. We are the Temples of the Holy Spirit. Our thoughts, our actions, our desires, are by rights more his than our own. But we have to struggle to ensure that God always receives from us what we owe him by right. If we do not labor to overcome our natural weakness, our disordered and selfish passions, what belongs to God in us will be withdrawn from the sanctifying power of his love and will be corrupted by selfishness, blinded by irrational desire, hardened by pride, and will eventually plunge into the abyss of moral nonentity which is called 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.