If you follow this advice, if you first recognize that you are a child of wrath by nature, guilty of eternal death and damnation, from which no creature, either man or angel, is able to save you, and if you then grasp God’s promise, believing that he… has sent Christ, his only Son, to render satisfaction for your sin, to give you his innocence and righteousness, and finally to redeem you from all danger and death, then do not doubt that you belong to the little flock of the elect.
~Martin Luther~
Historical Theology (Grand Rapids, Michigan; Zondervan; 2011) p.461
The Old Guys 
Where is this reference in Luther? (Not Grudem.)
Thanks!
Or Allison.