Here I sketch out the intricacies of the concepts of being and becoming elicited from discussions generated by Underhill's text. In order to appreciate and elucidate being–becoming in its relevance to mysticism, I shall read that pairing through the lens of dialecticism. My contention is that being and becoming are not mutually exclusive. Rather, they are dynamically related to one another in a dialectical fashion. The fundamental premise of theistic mysticism, as consistently emphasized by Underhill, is the intimacy between the finite mystic and infinite reality. This chapter introduces the dialectical triad of becoming–being–infinite being, which forms the metaphysical foundation of theistic mysticism and its development. The chapter has recourse to Bergsonian vitalism in order to highlight possible states of affairs that allude to a transcendence–immanence relation. It also introduces Hegel's mediating and individualizing dialectics, which will form the "dialectics' component of this work's interpretive instrument. The application of Hegel's dialectics will show up three different models of transcendence with two of them taking the forms: infinity's incorporating and overflowing the finite and infinity's otherness to the finite. Mysticism as an encounter with infinite being has correlative features with the sublime. Sublimity as theorized by Kant offers us an apposite hermeneutic tool for reading mysticism. Hence, major areas within this chapter will be devoted to framing Kantian sublimity and epistemology against testimonies of mystics claiming to experience God as the two forms of transcendence stated above.