This dissertation investigates exactly what role the proofs for the existence of God, and rational theology more widely, played in the history of modern philosophy from Descartes up to our own day. If the proofs are poorly suited to the role of religious apologetics, just as little can they be read as straightforward philosophical “idolatry.” I argue that a better understanding of the proofs requires an exhaustive rereading of, and expansion upon, Kant’s approach to rational theology as it develops over some four decades. The middle of this dissertation effects just such a rereading, beginning from Kant’s earliest works and advancing towards his moral theology and his critique of speculative metaphysics. One of the discoveries of this rereading is that what Kant calls “rational faith,” a central concept for him, will intersect with what Heidegger calls “ontotheology.” I then argue via Kant that the proofs, as explicit theoretical arguments, can be reinterpreted as mere symptoms generated by more fundamental practical needs. These needs, and rational faith, can then be reread through Heidegger as what I call a “practical ontotheology.” One effect of this is that it demonstrates the importance of reexamining the interests of metaphysics more broadly, except in practical (and not theoretical) terms.