Hugo spills a bit of ink here letting us know that he believes that in many ways, convents that still existed during his day (like the one described in the previous chapters) could be harmful relics of the past. He illustrates this by describing his perspective of the actual reality of the nuns in Spanish convents like the one in Petit-Picpus:
…They are women who believe themselves wives; ghosts who think themselves seraphim. Do they desire or love or live? No. Their nerves have turned to bone, and their bones are rock. Their veils are woven night, and the breath within them is like the breathing of death.
Dipping into a bit of an interpretive mode here and reading between the lines, I think Hugo is pointing out that though at the outset the creation of convents and monasteries may have been all about fostering a deep connection to the infinite, it has now functionally erected a barrier between the infinite and humanity at large. The picture of the Infinite that is being broadcast on the human canvas here is one that is distorted and malformed. It does not draw one toward love and life, but instead chokes and obscures those things.
For Hugo, there is something beyond, but that something is not being accurately reflected by the lived practice and unchanging ways of the convent.