"As the flame of a fire has three qualities, so there is one God in three Persons. How? A flame is made up of brilliant light and red power and fiery heat. It has brilliant light that it may shine, and red power that it may endure, and fiery heat that it may burn. Therefore, by the brilliant light understand the Father, Who with paternal love opens His brightness to His faithful; and by the red power, which is in the flame that it may be strong, understand the Son, Who took on a body born from a Virgin, in which His divine wonders were shown; and by the fiery heat understand the Holy Spirit, Who burns ardently in the minds of the faithful. But there is no flame seen where there is neither brilliant light nor red power nor fiery heat; and thus also where neither the Father nor the Son nor the Holy Spirit is known God is not properly worshipped."
Hildegard of Bingen in An Anthology of Christian Mysticism