The French philosopher Alain Badiou (born 1937), is one of the main representatives of a recent philosophical homage to Saint Paul. Yet, Badiou is not a believer in the traditional sense, let alone a Christian philosopher. On the contrary, he rejects transcendence and pleads for a radical this-worldliness. This does not mean however that his work is of no use for theologians, though a theological engagement with him will necessarily take some time. This book takes the first steps in that direction. It focuses on Badiou’s ontology because his challenge to theology, and more in particular to the doctrine of God, is to be found at this level of his system. The starting point is Badiou’s claim that nowadays true religion and true faith are no longer possible. To evaluate this claim, three steps are taken. In a first step, taken in Chapter 1, the theological context in which Badiou’s meta-ontology can become of interest is investigated. Starting from a clarification of the meaning of the terms ‘religion’ and ‘faith’, the issue of the existence of God is examined, which will result in the conclusion that true faith and true religion ask for a proof for the existence of God if we want to avoid what will be designated as the ‘closed circle of faith presupposing faith’. This leads us to the necessity for an ontology which repeats what Aquinas has done within the medieval-Aristotelian world-picture of his age, namely offering an openness towards God. Chapter 2 opens with an argument as to why we should turn to Badiou as a plausible source for such an ontology and will then continue by investigating the basic elements of his meta-ontological system as they are presented in the first part of Being and Event: his mathematical turn, his decision on the non-being of the one, his declaration of the void and the actual infinite. Chapter 3, finally, examines whether, despite Badiou’s outspoken atheist stance, his ontology (expressed through set theory) cannot be opened towards God. For this, the book falls back on Georg Cantor, the founder of set theory, who, in contrast to Badiou, did not draw from set theory the atheist conclusion that God is now really dead.