…only unbelief is called sin by Christ, as he says in John, chapter 16, “The Spirit will punish the world because of sin, because it does not believe in me.” Furthermore, before good or bad works happen – which are the good or bad fruits of the heart – there has to be present in the heart either faith or unbelief, the root, sap and chief power of sin. That is why, in the Scriptures, unbelief is called the head of the serpent and of the ancient dragon which the offspring of the woman, i.e. Christ, must crush, as was promised to Adam.
– Martin Luther
as quoted by Tchividian, Tullian; One Way Love: Inexhaustible Grace for an Exhausted World; David Cook Publishers; copyright 2013; Kindle Edition; Location 2315