The act of having true faith in Christ is enough justification in Luther's mind to be a Christian. He uses Biblical excerpts to prove this doctrine of "Justification by Faith Alone." Luther identifies with Paul from Romans 10, "for man believes with his heart and so is justified." (10) Accordingly, if only faith can justify the faith of a Christian, then no outer work needs to be performed to justify one's Christianity. The only action that makes you less Christian is when you begin to doubt your faith and start questioning Christ's divinity. Faith in Christ is so strong that it is considered to be the "perfect fulfillment of the law, will fill believers with so great a righteousness that they will need nothing more to become righteous." (10)
The Word of God further backs up Luther's doctrine. This is portrayed most obviously when Luther again recites Sacred Scripture, "If you wish to fulfil the law and not covet, as the law demands, come, believe in Christ in whom grace, righteousness, peace, liberty, and all things are promised to you.
If you believe, you shall have all things; if you do not believe, you shall lack all things." (11-12) Furthermore, Luther adds that faith is enough since most of the commandments are meant to teach us what we are incapable of doing. For instance, the commandment "You shall not covet" is impossible to obey. These commandments simply show man their imperfections and to further fortify the glory of what Christ was capable of.
In addition, faith alone makes the Word of God effective. Faith is the only way that the Word of God is preserved. Thus, "it is clear that, as the soul needs only the Word of God for its life and righteousness, so it is justified...