 In 1623 and 1625, Mersenne published three massive philosophical works. The Questiones celeberrimae in Genesim (which covers almost 1900 folio columns), a commentary on the six first chapters of the book of Genesis, larded with long scientific and philosophical digressions, was intended as an apology of Christian religion against all forms of “atheism”. It contains attacks on various Platonist, cabbalistic and hermetic authors such as Bernardino Telesio, Pietro Pomponazzi, Giordano Bruno, Jacques Gaffarel, or the English hermetic author, Robert Fludd. The tone is virulent, sometimes offensive, and the book attracted several outraged answers. Fludd in particular reacted in his Sophia cum moria certamen (1629) which itself drew a long reply from Gassendi, writing upon Mersenne’s request (Gassendi 1630). The same inspiration governs L’Impiété des Déistes, published in French the next year. It is a point-by-point refutation of the 106 Quatrains du Déiste or Anti-Bigot, a long anonymous poem denouncing Christian revelation, bigoted superstition and belief in a personal God. Mersenne’s response, written as a dialogue between a deist and a theologian, leads to the conversion of the deist, to whom the existence, justice and mercy of the Christian God are demonstrated. Dedicated to Richelieu, and perhaps written on command, the book may be seen as taking part in a larger campaign orchestrated by the French state and the catholic church, at a time that witnessed Vanini’s execution in Toulouse, the trial of Theophile de Viau, and a strengthening of repressive measures against anti-Aristotelian philosophical novelties. La Vérité des sciences (1625) is a dialogue between an alchemist, a skeptic and a Christian philosopher, the latter opposing both the over-confidence of the first and the excessive doubts of the second. The second, third and fourth parts of the book, dealing with arithmetic and geometry, is an 800-page compilation of ancient and recent achievements in the mathematical sciences, intended to show that there are regions of human knowledge immune from doubt, and which do not contradict Christian wisdom.