‘God is simple’, wrote Messiaen; here is an aesthetic model for this composer – sometimes called a theologian – that is present in diverse aspects of his music and that receives attention in this chapter. Simplicity is revealed in different forms and techniques, and in his ‘naïveté. It is so engraved in the composer’s works that it becomes almost a caricature. This chapter examines these ideals in Messiaen’s thought, character, and in the critical reception of his music.