Olivier Messiaen’s only opera St François d’Assise to his own libretto (based on medieval sources) is something of an anomaly among operatic works. It was mainly written between 1975 and 79 and first performed at the Paris Opera House in 1983. Although Francis of Assisi’s life contains plenty of dramatic moments and could well have given rise to a narrative dramatic account, Messiaen – consciously – chose an almost opposite approach. The opera is divided up in 3 acts, further subdivided into 8 tableaux, the notion of tableau providing a key to the meaning of the work. Different from most operas, St François d’Assise has little that could be termed action and is only concerned with spiritual contemplation and moments of inner combat from St Francis’ life. In other words, the work functions as a kind of meditation on St Francis and his particular spirituality, pointing to a fundamental point of Franciscan piety: the conformity of St Francis with Christ. Messiaen, well-known as a devout Catholic, thus chose to use the operatic stage as a place for religious contemplation. I shall argue that Messiaen, through musical, textual, and scenic means also involving visual medievalisms in some measure, altogether appropriated what may be characterized as a Franciscan way of taking his religious message to places where people assemble, also outside of the church: as the Franciscans would preach on street corners and market places. Messiaen preaches, as it were, in the opera house, just as he, in many works, may be said to have done in the concert hall. I shall discuss only certain aspects of this huge opera, contextualizing it with other modern religious musical works drawing on medieval traditions, and with the aim to discuss the notion of medievalism and its dependency on ideas of chronology and/or ideology.