I want to draw attention to the fact that Luther does not talk about what constitutes theology but about what makes a theologian. This is somewhat characteristic of his approach: many people have noted the importance of his "theology of the cross," which he articulated most dramatically at the Heidelberg Disputation in 1518; but the text of the disputation theses do not speak of a theology of the cross; rather they speak of a theologian of the cross. Theology, for Luther, is the words spoken by human beings in response to the words God has first spoken to them; thus, theology is a personal action; and therefore, there can be no discussion of theology without first discussing the agent, the one who speaks theologically. Theology is an abstraction unless it is understood as the action of the theologian.
It's not first and foremost an intellectual exercise! The failure to realize that is what distresses me about some of the theology I read from time to time.