Chapter 2, Part 2 - Criticism of the Roman Mass: Opus Operatum (p. 65).
Luther was likely one of the most conservative Reformers. His development of ideas was slow and appeared piecemeal. On p. 66 Sasse says that Luther gave up opus operatum, then the sacrifice element of the mass, then communion in only one kind (element), then transubstantiation. Whenever possible, he kept what he could unless Scripture demanded otherwise. As an example of Luther’s development, Sasse traces his statements on the Mass as our offering a sacrifice to God. He is careful to discuss the practice and its implications. In the end, he concludes that we are unable to and unworthy of offering any acceptable sacrifice to God, but that God gives us every good gift. This is a more clear and focused rejection than we see in the more radical Reformers.