Gregory the Great — on Ezek 1:5 (Homilies on Ezekiel, Book 1, Homily 2)

“"And in the midst of it the likeness of four living creatures." What is said to be in the midst of it, whether of electrum or of fire, nothing prevents us from understanding, because these four living creatures, namely the holy evangelists, were both strengthened in the virtue of faith from the incarnation of the same Lord, and afflicted with many tribulations in the fire of persecution.”