The identity of Islamic denominations is closely tied to the Quran and its exegesis. Moreover, the core identity of the Quran is nothing but monotheism. In this research, we deploy the library method to study prominent, selected Shiite, Ashʿ arite, and Muʿ tazilite exegeses of the Quran, establishing that the most authentic idea and process of approximation between Islamic denominations is the monotheistic theory of approximation. Accordingly, just like all other Islamic doctrines, approximation is originated from monotheism, figuring in a world of concepts with a central role by monotheism around reason and wisdom. This theory has at least two outcomes: first, it makes no sense to negotiate over explicit monotheistic texts, but as for apparent meanings and ambiguous phrases of religious texts, one might seek harmony and reconciliation. Moreover, differences result from contradictions or conflicts. As long as such differences do not contradict the belief in monotheism, they are included in the scope of approximation and reconciliation, and when such a contradiction occurs, approximation makes no sense.