That task [of constructing theology according to Luther's principle] I essay in the full consciousness that my action is justified and rendered imperative by the standard writings of the Reformation. But if we can rightly know God only if we know Him through Christ, then we can know Him only if we belong to the community of believers. Not only, however, are God and all the operations of His grace to be construed through the revelation in Christ, but even sin can be appreciated only in virtue of the forgiveness of sins which is Christ's special gift.
Need another hint? The same author penned these lines a few paragraphs later:
[I]f anyone builds Christian theology on a substructure of pretended Natural Theology, the rationalistic arguments of Augustine about original sin, and those of Anselm about the nature of redemption, he thereby takes his stand outside the sphere of regeneration, which is coterminous with the community of believers.