 Apart from its polemical intent, Mersenne’s “scientific apologetics” may be seen as articulating a natural theology together with a “sacred physics” or “sacred mathematics”. On the one hand a host of rational arguments—some of them borrowed from the sciences, some from traditional metaphysics—are brought in to prove God’s existence and demonstrate His attributes. This natural theology is where Mersenne articulates his main ideas on metaphysics and we shall come back to it in the next part of this article. On the other hand, science is used as an instrument of exegesis. The Quaestiones in Genesim contains thus numerous scientific excursuses whose aim is either to give a scientific (mainly mathematical) exposition of a biblical content—as when Mersenne “calculates” what should be the size of Noah’s ark, or suggests that our knowledge of architecture should help us to see the excellence of the temple of Solomon—or to illustrate a Catholic mystery (such as the triadic unity of God or the miracle of the Creation) through a mathematical analogy.