Gallagher's research uses the development of the early Christian movement in the Greco-Roman world as a paradigm of how an initially obscure and suspicious cult movement can enter the social mainstream and achieve a position of power and respectability. His familiarity with early Christianity decisively shapes his understanding of contemporary new religions, or so-called "cults." In both instances he is particularly interested in forming an understanding of the processes of conversion and disaffiliation as well as a group's missionary activity and self-defense ("apologetics").