Jesus describes the principle on which they might give the church a name. He had taught that they should pray to the Father in his name, and that in his name they were baptized. The people who were baptized were to take upon them his name. In other words, they were to be identified with him. It would be something similar to a child that is adopted into a family and takes the family name. They become identified through the family name.
Thus, Jesus notes that, “how be it my church save it be called in my name?”