Presenting definitions for (1) theoretical and practical as well as (2) instrumental and non-instrumental rationality, this paper provides a mechanism for defending the practical rationality of commitment to religious teachings or religiosity. First, relaying on the Abraham Maslow's theory of hierarchy of needs and the theory of Thaddeus Metz and Susan Wolf on the meaning of life, the rationality of “meaningfulness of life” as a fundamental goal in human life has been advocated. Thus, based on the descriptive viewpoint of Ninian Smart, the contemporary phenomenologist of religion, about the various dimensions of religion, the paper provides a mechanism for defending the practical instrumental rationality of religiosity. It is shown that if religions, regarding their various dimensions, be able to provide a certain level of human needs in Maslow's theory, and give meaning to life of believers according to Metz and Wolf, then defending the instrumental rationality of religiosity would be able. The need to the theoretical rationality of religious teachings to defend the instrumental rationality of religiousness; the comprehensiveness of the concept of "meaning of life" as the goal of religiosity in explaining the practical rationality of religiousness, compared to other purposes of religiousness; the ability of religions to give meaning to human life, or at least to the lives of believers, as one of the criteria ;