Saint Gregory the Dialogist
You have heard that the Holy Spirit descended upon the disciples in tongues of fire. But why did the Holy Spirit, co-eternal with the Father and the Son, appear as fire? Why as tongues of fire? Why does He sometimes appear as a dove and at other times as a flame? Why did He descend as a dove upon the Only-begotten Son, but as a flame upon the disciples? The Holy Spirit appears as fire because, in the words of the holy Apostle Paul, God is an ethereal, indescribable, and invisible fire: “For our God is a consuming fire” (Heb. 12:29). God is also called fire because He destroys the rust of sins.
But the Lord sends fire down to earth when with the breath of the Holy Spirit He enflames the hearts of mortal men. And the earth lights up when a heart of flesh, that is cold in its iniquitous pleasures, abandons the desires of the modern world and becomes enflamed with love for God. Thus the Spirit very properly appeared as fire, because from every heart in which He chooses to dwell, He dispels the numbness of ice and enflames such a heart with a desire for eternal life. And the reason He appeared in tongues of fire was because the Spirit is co-eternal with the Son, while the Son is the Word of the Father; thus, just as the tongue is in close proximity to the word, so the Holy Spirit appeared in flaming tongues, in order to show the Spirit’s particular closeness to the Word of the Father. Furthermore, the Spirit also appeared in tongues of fire because He causes everyone in whom He dwells to be ardent and cogent.