I think it is probably a point which many do not realize; that in France it was the custom for the organ to ‘fill in’ around the Propers of the Mass; and even, in certain very specific circumstances, to substitute for them – there was not, for instance, an Offertorium or a Communio in the Mass of the Easter Vigil, so the Organ was traditionally played instead.
As a result, the music had to form an integral part of the liturgy. Messiaen did not, perhaps, approach this aspect of it in quite the same way as Charles Tournemire, who wrote a complete set of ‘improvisations’ to fit every Sunday and major Feast Day of the year (Advent & Lent excluded, of course !) - his improvisations were exactly that, and were largely left unrecorded; so that there is little record of his playing on the hundreds of Sundays on which he occupied the ‘banc des Grands-Orgues’ at the S. Trinité in Paris (he was Titulaire there for some 61 years !).
However, what we do have is many major works which have great spiritual dimensions, and which can even form enormous spiritual experiences of themselves : Apparition de l’Exglise Eternelle, for instance, is his vision of the Heavenly Church; whilst Joie et Clarté des Corps Glorieux represents the joy and limpidity of the glorious bodies of the Resurrected. (I should say it is perhaps my favourite of all his pieces, and I used to delight in it during communion every Easter Vigil for many years as an Anglican).
If you are not familiar with Messiaen’s organ music, I suggest that you consider listening to some . . . I did try to link some in to this post, but failed miserably; so forgive me for that, but you can find a lot of information about him, and some links to his music, on his Wikipedia page.