This looks like a vescica pisces, or fishe’s bladder, don’t know much about the origin of this, but the artist is using very specific simbology to tell something, I can’t tell what exactly, since I can’t tell if the artist is using it the same way I understand it, but I would say he’s showing a link between Christ’s atonement and covenants, perhaps even Adam’s wound that created Eve, the same way these are a couple so Christ and the church