In logic, all it takes is one single contrary to negate an assertion.
If I make the statement, "all cheese is green," and you hand me a slice of Velveeta, you've proven me wrong.
(Which begs the question of whether Velveeta is cheese. But that's for another day).
The phrase "we are by nature sinful and unclean" is used in the confession in several liturgies in the Lutheran Service Book. On its face, the phrase is saying that human nature is by nature sinful. In other words, part of "being human" is to "be sinful."
However, 3 individuals have been human without being sinful. Adam and Eve in their pre-fall state were human, but not sinful.
But more compelling is that Christ has had a human nature from the moment of conception in the Virgin, a nature He still has.
Christ is like us in all ways without sin: a man who was thirsty, tired, sleepy, hungry.
And what's more germane to this discussion is that His being a man did not make Him a sinner, and His not being a sinner didn't make Him less of a man.
Philosophically speaking, sin is an "accident" of human nature, something that we have, but something that's not of the essence of being human. 200 years from now, none of us will be sinner, except as a historical factor. But we will still be fully and completely human.
I think I know what this phrase is trying to say: that we are shot through with sin, that sin isn't just a minor problem, but something that messes up our lives on every level.
All of that is true. But phrased the way it is, this is a very dangerous statement, one which impinges on a correct Christology.
I wish this were an example of what theologians call an "inadvertent error": one that's just crept in accidentally and temporarily. It's not. It's used in several liturgies in an official service book for the LCMS.
What I'm trying to figure out is what this tells us about our confession. Is our anthropology (our doctrine of man) messed up? Even more important, is our Christology messed up?
I'm more and more convinced that what goes on in our services and liturgies and hymns tells us more about our confession than we would care to admit. It doesn't suffice to say that this should be corrected. It should. The bigger question is how such an obvious and dangerous statement got there in the first place.