It is, and it is the hardest canticle of the Comedy, and also the least remembered. There is hardly any plot, the body and blood of the Inferno give way to light, music, dance, and number, and whoever enters expecting a description of heaven finds lessons. The difficulty comes less from a hidden idea than from the poem's demand: patience with arguments that Beatrice and the saints unfold canto by canto.
Dante himself warns, in canto II, that the sea ahead is deep. He asks those who follow in a small boat to return to the shores they know, because on the open sea they may get lost, and he invites only the few who raised their necks early to the bread of the angels. Beyond the warning, the references weigh: the cosmology of the heavens, scholastic theology, the politics of the thirteenth century and of 1300, the names of people today's reader does not know. An annotated edition helps a great deal.
Someone who has never read it can begin with the first group of cantos, the Moon, and then follow a short route: the prophecy of exile, the ladder of Saturn, the examination of faith, and the final vision. Someone who wants rhythm reads one canto at a time, asking in each who is speaking and what the pilgrim manages to see. And whoever reads slowly discovers that the poem treats difficulty as a subject: at every height the poet confesses that language does not reach, and that confession is part of the reading.