When the fiftieth day after the Resurrection had come, the day we now commemorate, all the disciples were gathered with one accord in the upper room, each having also gathered together his thoughts (for they were devoting themselves intently to prayer and hymns to God). "And suddenly", says Luke the Evangelist, "there came a sound from heaven as of a rushing mighty wind, and it filled the house where 'they were sitting" (Acts 2:1-11). This is the sound which the Prophetess Hannah foretold when she received the promise concerning Samuel: "The Lord went up to heaven and thundered; and he shall give strength and exalt the horn of his anointed" (cf. 1Sam. 2: 10 Lxx). Elias' vision also forewarned of this sound: "Behold the voice of a light breeze, and in it was the Lord" (cf. 1Kgs.19:12 Lxx). This "voice of a light breeze" is the sound of breath. You might also find a reference to it in Christ's Gospel. According to John the Theologian and Evangelist, "In the last day, that great day of the feast", that is to say Pentecost, "Jesus stood and cried, saying, If any man thirst, let him come unto me and drink... This spake he of the Spirit, which they that believe on him should receive" (John 7:37-39). Again, after His Resurrection He breathed on His disciples and said, "Receive ye the Holy Ghost" (John 20:22).
That cry of Christ prefigured this sound, and His breathing upon the disciples foretold the breath, which is now poured down abundantly from above and resounds with a great voice heard far and wide, summoning everything under heaven, pouring grace over all who approach with faith and filling them with it. It is forceful in that it is all-conquering, storms the ramparts of evil, and destroys all the enemy's cities and strongholds. It brings low the proud and lifts up the humble in heart, binds what should not have been loosed, breaks the bonds of sins and undoes what is held fast. It filled the house where they were sitting, making it a spiritual font, and accomplishing the promise which the Saviour made them when He ascended, saying, "For John truly baptized with water; but ye shall be baptized with the Holy Spirit not many days hence" (Acts 1:5). Even the name which He gave them proved to be true, for through this noise from heaven the Apostles actually became sons of Thunder (cf. Mark 3:17). "And there appeared unto them", it says, "cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Spirit, and began to speak with other tongues, as the Spirit gave them utterance" (Acts 2:3-4).
Those miracles accomplished by the Lord in the flesh, which bore witness that He was God's only-begotten Son in His own Person, united with us in the last days, came to an end. On the other hand, those wonders began which proclaimed the Holy Spirit as a divine Person in His own right, that we might come to know and contemplate the great and venerable mystery of the Holy Trinity. The Holy Spirit had been active before: it was He Who spoke through the prophets and proclaimed things to come. Later He worked through the disciples to drive out demons and heal diseases. But now He was manifested to all in His own Person through the tongues of fire, and by sitting enthroned as Lord upon each of Christ's disciples, He made them instruments of His power.
Why did He appear in the form of tongues? It was to demonstrate that He shared the same nature as the Word of God, for there is no relationship closer than that between word and tongue. It was also because of teaching, since teaching Christ's Gospel needs a tongue full of grace. But why fiery tongues? Not just because the Spirit is consubstantial with the Father and the Son - and our God is fire (cf. Heb. 12:29), a fire consuming wickedness - but also because of the twofold energy of the Apostles' preaching, which can bring both benefit and punishment. As it is the property of fire to illuminate and bum, so Christ's teaching enlightens those who obey but finally hands over the disobedient to eternal fire and punishment. The text says, "tongues like fire" not "tongues of fire", that no one might imagine it was ordinary physical fire, but that we might understand the manifestation of the Spirit using fire as an example. Why did the tongues appear to be divided among them? Because the Spirit is given by measure by the Father to all except Christ (John 3:34), Who Himself came from above. He, even in the flesh, possessed the fullness of divine power and energy, whereas the grace of the Holy Spirit was only partially, not fully, contained within anyone else. Each one obtained different gifts, lest anyone should suppose the grace given to the saints by the Holy Spirit was theirs by nature.
The fact that the divine Spirit sat upon them is proof not just of His lordly dignity, but of His unity. He sat, it says, "upon each of them. And they were all filled with the Holy Spirit" (Acts 2:3-4). For although divided in His various powers and energies, in each of His works the Holy Spirit is wholly present and active, undividedly divided, partaken of while remaining complete, like the sun's ray. They spoke with other tongues, other languages, to people from every nation, as the Spirit gave them utterance. They became instruments of the divine Spirit, inspired and motivated according to His will and power. Anything taken hold of by somebody outside itself, sharing in the energy but not the essence of the one acting through it, is his instrument. As David declared through the Holy Spirit, "My tongue is the pen of a ready writer" (Ps. 45:1). The pen is the writer's instrument, sharing in the energy, though obviously not the essence, of the writer, and inscribing whatever he wishes and is able to write.
St. Gregory Palamas - Homily 24 - On How the Holy Spirit was Manifested and Shared Out at Pentecost - ( Paragraphs 4-8 of 18 ) - St. Tikhon's Seminary Press