Our Diseased Reason 

The Gospel is a sort of teaching which cannot, like the teaching of the Law, be understood by reason, but with which reason utterly disagrees. For reason cannot comprehend a forgiveness of sins for the sake of a foreign merit nor a righteousness that is foreign. Moreover, not even in the saints is this knowledge firm enough. For although we condemn reliance on works and superstition, yet we are not able to eject these entirely from the heart; but they are a sort of disease born into the flesh -- a disease which plagues the spirit more severely than any lust plagues the flesh.

Source: Quoted in Ewald M. Plass, What Luther Says, A Practical In-home Theology for the Active Christian. St. Louis: Concordia Publishing House. 1959 Edition, 10th Printing (1994), p. 566.