Luther's Gospel Struggles 

The devil caught me by surprise a few times, when I had lost sight of this chief article.  He tortured me so badly with passages of Scripture that heaven and earth became too small for me.  Then human works and laws seemed to be all right, and nothing at all was wrong in the papacy.  In short, it seemed that on one had ever erred except this fellow Luther and that my best works, my doctrine, preaching, and books must be accursed.  Moreover, even the abominable Mohammed almost became a prophet to me, and both Turks and Jews turned to very saints.  Therefore, dear brother, do not be proud, or too sure and certain that you know Christ well.  You now hear me confessing and professing what the devil was able to do against this man Luther, who, after all, really ought to be a doctor in this art.  He has preached, thought, written, spoken, sung, and read so much about his matter yet must remain a pupil in it and at times is neither pupil nor master.  Therefore be advised, and do not shout hurrah.  Now you are standing, but see to it that you do not fall.  

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.