Trusting Grace
is Hard

To speak about the matter [of forgiveness of sin] is easy; but in the hour of temptation it is very difficult to hold with certainty of heart that we possess forgiveness of sins and peace with God through grace alone, to the exclusion of all other means in heaven and on earth....

But conscience cannot be brought to rest and joy unless it has peace through this grace, that is, through the forgiveness of sins promised in Christ. Therefore there will be no peace for my bones and yours unless we hear the word "grace" and cling to it steadfastly and faithfully.  But then our conscience will surely find peace. 

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. 610.