Is unbelief sin or does it just lead to sin? In reading John 3:16, I know that if we do not believe in Christ we can not be saved in the Kingdom of God.
Unbelief in and of itself is a word describing those who have either chosen or who have been raised without belief.
If our traditional understanding of sin is accurate, sin is acting knowingly against God’s commandments, then those who have chosen unbelief are indeed sinning. Children who have been raised as atheists, even agnostics, I would believe they are not in sin until they have sufficiently been taught, and have chosen themselves to continue in unbelief. Unbelief, however, invariably leads to sin.
Yes, we cannot be saved unless we have faith, or believe, in Jesus Christ. Some people will not find salvation until they are taught in the Spirit World and they accept these teachings.