And there appeared unto them separated tongues like as of fire, and it sat upon each of them.
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Cyril of Jerusalem
And there appeared to them separate tongues:
Thus they were entirely baptized according to the promise, and invested soul and body with a divine garment of salvation. They partook of fire, not of burning but of saving fire; of fire which consumes the thorns of sins, but gives lustre to the soul.
Tongues of fire. The Hebrews use the name tongue, for almost any thing pointed. Thus they say, a tongue of the earth, for a promontory. (Josue xv. 5.) A fiery tongue for a flame in shape of a tongue. (Isaias v. 24.) The expression, therefore, in this place, may mean noting more than sparks, or rather flames, which appeared above all who were in the house.
By the fiery tongues is signified the efficacy of the apostles' preaching, and the gift of tongues bestowed upon them. (Menochius) ...