Martin Luther wrote the following
Only unbelief is called sin by Christ. Christ says in John (16:9-10), “And when he (the Holy Spirit) comes, he will convict the world concerning sin and righteousness and judgment: concerning sin, because they do 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, which is the root, sap and chief power of all 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, Christ, must crush, as was promised to Adam (cf. Gen. 3:15).
The words “grace” and “gift” differ in that grace actually speaks of God’s kindness or favor which he has toward us and which leads God to pour Christ and the Spirit with his gifts into us. The gifts and the Spirit increase daily in us, yet they are not complete, since evil desires and sins remain in us which war against the Spirit.
Genesis (3:15), proclaims that the offspring of the woman and that of the serpent will be enemies through Christ. Through grace of Christ we are made right before God. God’s grace is not divided into bits and pieces, as are the gifts he gives, but grace takes us up completely into God’s favor for the sake of Christ, our intercessor and mediator, so that the gifts may begin their work in us.
—Martin Luther, Preface to Romans