Unbelief is not sin in itself. Sin is the exertion of will (decision, determination, action) in opposition to God. Unbelief is a gut reaction, an assessment of God's character and his word. Adam and Eve did not believe God nor what he said to them, and the result was choice exerted in opposition to God. The unbelief wasn't the sin, the action it led to was. Unbelief is a state of heart and mind in regard to God which leads to sin.
Unbelief, therefore, is not a sin that can be condemned or forgiven, though the sin it leads to certainly can be. Unbelief itself is merely the spiritual and soulish framework from which sin arises, but not sin itself. As such, neither faith nor unbelief can properly be thought of as works as sin can. They underlie works but are not works themselves.
Whereas it is true that a sinful heart is unbelieving, and it is an undeniable consequence that an unbelieving heart will be sinful; sinful and unbelieving are not synonymous. They are interrelated but not equivalent. Whatever is not of faith is sin, which I think is about the same as saying whatever is of unbelief is sin. The sin is not the unbelief, the unbelief is not sin, sin is what happens when one acts without faith.
To those who argue that if Arminian tenets are true, faith is a work and unbelief is a sin which cannot be forgiven, I say think again! Unbelief is only a state which keeps one from entering into God's promises. It is not unbelief that results in judgment, but unbelief that keeps one under the condemnation for their sin. So, unbelief is not a sin for sin is an act of the will, but unbelief is the state of heart and mind that produces sin and keeps one under the judgment for it.
