For the words of the preachers are rightly called "sparks," because they set ablaze those whose hearts they have touched. We must also consider that sparks are extremely fine and delicate, because when the sacred preachers speak of the Heavenly Kingdom they do not avail to reveal in words so much as inflame with desire. Therefore, their tongue comes to us, as it were, as sparks, because in their voice something subtle from the Heavenly country is recognized with difficulty, something which is, however, loved by them without subtlety. Then, they neither suffice to see the Heavenly glory as great as it is, nor can they articulate as much as they see. Therefore, the glowing brass emits sparks when the preacher is barely able to declare faintly that from which he himself is strongly kindled. For he is moved by Divine piety, so that from these selfsame frail sparks, the spirit of the hearer is inflamed, because there are those who hearing trifles are replenished with greater desire and thence burn perfectly with love for God when they have hardly received the most tenuous sparks of the words.
Gregory the Great, preaching on Ezekiel 1:7