I guess that 'medium' refers to the material on which a work of art has been made. 'A miracle is being worked' plays on the dual meaning of 'work' in this context: the working out of a miracle, and the creation of a work of art. Paper, card, wood, fabric, etc are examples of media in the production of artwork. Each of these is a 'medium' [a countable noun]. The particular medium referred to is one that may quickly dissipate, making it unlikely that the artwork will endure - that would be a miracle.
All the above is guesswork on my part, and the reference may be quite different, but the logic of the expression would still apply.