Mereological pantheism is the view that the pantheist thesis, God is identical with the universe, is characterised by mereological notions and defended with arguments from mereology. In this paper, we shall argue that mereological pantheism is not a tenable theory because of its endorsement of the following three theses. (i) Existence pluralism, i.e. there is a plurality of things. (ii) Things are embedded with certain mereological structures and the mereological structures are presumably characterised by classical extensional mereology. (iii) Composition is a kind of identity. We argue that the three theses are jointly inconsistent. Moreover, if mereological pantheism denies the first assumption, it collapses into theism or atheism. Rejecting the second assumption betrays the name of “mereological pantheism.” And if the third assumption is denied, there is a gap between the claim that God is the composition of all things in the universe and the claim that God is identical with the universe. Thus, mereological pantheism is not a tenable version of pantheism.