Article XIX. The Cause of Sin
It is taught among us that although almighty God has created and still preserves nature, yet sin is caused in all wicked men and despisers of God by the perverted will. This is the will of the devil and of all ungodly men; as soon as God withdraws his support, the will turns away from God to evil. It is as Christ says in John 8:44, "When the devil lies, he speaks according to his own nature.
In our previous comments on the freedom of the will, we concentrated more on the concept of freedom in the Spirit and less on the actual content of will and willfulness. This article directs our focus away from freedom and towards a (doubly bound?) will. The crucial point is made in the 2nd to the last sentence. The perverted will is the will of the devil; the true human nature is only truly human at the support of God, but when God withdraws support from the natural will, the will "turns away from God to evil."
The article is very clear in its connection to a common Lutheran understanding of the will as either ridden by Christ or the devil. There is, in this understanding, no autonomous will (the horse) that looks around at the available cast of jockeys (the devil or Christ), and then picks one or the other, or alternatively, choosing the natural and healthy alternative, rides off into the meadows free of any rider at all. No, the will is ridden, and the only remaining question is, "By whom?"
This may further answer our questions around the free will. We might say this: the free will is the will free from sin because it is supported by (and in) God. The bound will is the will in sin, because it is bound by (and in) the devil. The bound and the free will have this in common, that they keep company with someone. They separate on the issue of which company.
I notice here an almost synonymous use of the terms "nature" and "will", and wonder if that is the intent or understanding of the confessors.