Our Struggle Clinging to the Gospel

Justification is hard to hold (lubrica est), not indeed in itself -- for in itself it is most sure and certain -- but so far as our relation to it is concerned.  I often experience this myself, for I know the hours of darkness in which I sometimes wrestle.  I know how often I lose the roots of the Gospel and grace, as if they were suddenly hidden from me by dense clouds.  i know how slippery is the footing of even those who are experienced in this matter and can step up most firmly....  When in the present battle we should use the Gospel -- which is the Word of grace, consolation, and life -- then the Law -- the Word of wrath, sadness, and death -- anticipates the Gospel and begins to cause trouble; and the terrors which it raises in the conscience are no less than that terrible and horrible exhibition on Mount Sinai (Ex. 19:16), just as a single threatening verse of Scripture overwhelms and overshadows all consolation and so severely shakes all that is within us that we forget justification, grace, Christ, and the Gospel....  Moreover, half of ourselves is our opponent, that is we have our own reason and its powers against us.  In addition to this, the flesh, which cannot with certainty believe that the promises of God are true, resists the spirit.

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. 714