Open source development, to a great extent, is a type of social movement in which shared ideologies play critical roles. For participants of open source development, ideology determines how they make sense of things, shapes their thoughts, actions, and interactions, enables rich social dynamics in their projects and communities, and hereby realizes profound impacts at both individual and organizational levels. While software engineering researchers have been increasingly recognizing ideology's importance in open source development, the notion of "ideology" has shown significant ambiguity and vagueness, and resulted in theoretical and empirical confusion. In this article, we first examine the historical development of ideology's conceptualization, and its theories in multiple disciplines. Then, we review the extant software engineering literature related to ideology. We further argue the imperatives of developing an empirical theory of ideology in open source development, and propose a research agenda for developing such a theory. How such a theory could be applied is also discussed.