15 And lest men should be ignorant of the greatness of the mighty gift coming down to them, there sounded as it were a heavenly trumpet, For suddenly there came from heaven a sound as of the rushing of a mighty wind, signifying the presence of Him who was to grant power unto men to seize with violence the kingdom of God; that both their eyes might see the fiery tongues, and their ears hear the sound. And it filled all the house where they were sitting; for the house became the vessel of the spiritual water; as the disciples sat within, the whole house was filled.
Thus they were entirely baptized according to the promise, and invested soul and body with a divine garment of salvation. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them; and they were all filled with the Holy Ghost. 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. This is now coming upon you also, and that to strip away and consume your sins which are like thorns, and to brighten yet more that precious possession of your souls, and to give you grace; for He gave it then to the Apostles.
And He sat upon them in the form of fiery tongues, that they might crown themselves with new and spiritual diadems by fiery tongues upon their heads. A fiery sword barred of old the gates of Paradise; a fiery tongue which brought salvation restored the gift.
Source: Catechetical Lectures (New Advent)