As already pointed out, Holy Scripture describes original sin both a) as a defect, or lack of concreated righteousness (carentia iustitiae concreatae), and b) as concupiscence, that is, as a constant, vicious disposition to evil (habitualis inclinatio ad malum). This is taught in Rom. 7, 23: "I see another law in my members, warring against the law of my mind"; Gal. 5, 17: "The flesh lusteth against the Spirit"; etc. It is as concupiscence that original sin is something positive (positivum quid). However, sin is not positive in the sense that it is a material substance, which subsists of itself (substantia materialis, quae proprie subsistit). Original sin is not a substantia, that is, a self-existent essence, but an accidens, that is, an accidental matter, which does not exist by itself essentially, but inheres in a self-existent essence. Hence we must distinguish between human nature, which also after the Fall is the work of God, and the corruption of human nature, or original sin, which is the work of the devil.
This truth the Formula of Concord strenuously maintains against every form of Manicheism (Flacianism), which assumes two existent substances, of which one is essentially good and the other essentially evil. (Formula of Concord, Art. I. Augustine: "Original sin is not the nature itself, but an accidens vitium in natura, that is, an accidental defect and damage in nature." Thor. Decl., I, 55.)
On the other hand, our confession contends against Pelagianism and synergism with equal emphasis that original sin as an accidens is not "a slight, insignificant spot sprinkled or a stain dashed upon the nature of man or a corruption only in some accidental things, along with and beneath which the nature nevertheless possesses and retains its integrity and power even in spiritual things" (Thor. Decl., I, 21), but "such an unspeakable evil and such an entire corruption of human nature that in it and an its internal and external powers nothing pure or good remains~ but eversthing is entirely corrupt, so that on account of original sin man is in God's sight truly dead" (Thor. Decl., I, 60). Thus our Lutheran confession avoids both the Scylla of Manicheism and the Charybdis of Pelagianism.