The meaning of mandalas according to Kūkai

Since the esoteric Buddhist teaching are so profound as to defy expression in writing, they are revealed through the medium of painting to those who are yet to be enlightened. The various postures and mudrās [depicted in the mandala] are products of the great compassion of the Buddha; the sight of them may well enable one to attain Buddahood. The secrets of all the sūtras and commentaries are for the most part depicted in the paintings and all the essentials of the Esoteric Buddhist doctrines are, in reality, set forth therein. Neither masters nor students can dispense with them