Church organist is (and has been through history) a quite common profession - and a fair amount of well known composers and organists have held the position in specific churches: see https://en.wikipedia.org/wiki/Church_of_Saint-Sulpice,_Paris#List_of_organists_.5B26.5D and https://en.wikipedia.org/wiki/Notre_Dame_de_Paris#Organ_and_organists for examples