The setting and story of The Chronicles of Narnia are strongly linked to Christianity: Aslan, who sacrifices himself for a traitor in The Lion, the Witch, and the Wardrobe before returning to life, represents and literally is Jesus Christ, and much has been written about the Christian symbolism and allegory in the Narnia books.
How does this figure of ancient polytheism fit into the Christian story of Narnia? Of course, nature spirits like dryads and naiads are already not very consistent with Christianity, but it does feel consistent with the whole Narnian thing of talking animals. Bacchus, however, isn't a minor unnamed figure in Graeco-Roman mythology - he's one of the big twelve Olympian gods! How come he shows up as a minor figure in the Narnian mythos, apparently summoned by Christ/Aslan?