The essence of man is his freedom. Sin is committed in that freedom. Sin can therefore not be attributed to a defect in his essence. It can only be understood as a self-contradiction, made possible by the fact of his freedom but not following necessarily from it.
-Reinhold Niebuhr, The Nature and Destiny of Man, Volume 1 Human Nature (Up-per Saddle River, New Jersey:Prentice Hall, 1964), 17