Peter Steinke in Healthy Congregations: A Systems Approach, has a chapter called "The Immune Congregation." In this chapter he states: "The immune system is a network of cells that recognize and attack foreign invaders. The system asks one profound question: What is self, and what is not self?" [p. 91]

A little later he applies this insight:

The community needs an immune response, to determine what is self and not self. The community needs to ask, for instance, if a certain action continues, whether it will enhance the mission of the congregation or detract from it. Does an individual's or a group's behavior contradict or serve the congregation's purpose? Is there clarity about who is responsible for what and accountable to whom. [p. 91]

In a sense, that is what John does in vv. 19-28. He is both defining who he is and who he is not. He is clear about who he is and his mission. When he states that he is not the Christ and he is not Elijah and he is not one of the prophets, he is not saying that the Christ or Elijah or the prophets are bad; but simply that he is not them. Being clear about who he is and his mission, also means that he is clear about who he is not and what things will not contribute to his mission. While such an understanding of self (and non-self) is important for individuals, Steinke goes a step further and says that it is an essential part of being a healthy congregation.