First Reading: Isaiah 6:1-2a, 3-8
The Call of Isaiah
1 In the year that king Uzziah died, I saw the Lord sitting on a throne, high and lifted up; and his train filled the temple. 2 Above him stood the seraphim. Each one had six wings. With two he covered his face. With two he covered his feet. With two he flew. 3 One called to another, and said,
"Holy, holy, holy, is YHWH of Armies!
The whole earth is full of his glory!"
4 The foundations of the thresholds shook at the voice of him who called, and the house was filled with smoke. 5 Then I said, "Woe is me! For I am undone, because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for my eyes have seen the King, YHWH of Armies!"
6 Then one of the seraphim flew to me, having a live coal in his hand, which he had taken with the tongs from off the altar. 7 He touched my mouth with it, and said, "Behold, this has touched your lips; and your iniquity is taken away, and your sin forgiven."
8 I heard the Lord's voice, saying, "Whom shall I send, and who will go for us?"
Then I said, "Here I am. Send me!"
World English Bible
In the study for the Fourth Sunday in Ordinary Time (C), we investigated Jeremiah's prophetic call; today, we study Isaiah's call. Isaiah lived 150 years before Jeremiah. He was born into an aristocratic family who advised the King. He prophesied during the Assyrian advance on Jerusalem in 705 B.C. Indeed, much of his life as a court prophet (adviser to the king) was spent trying to keep Judah independent from Assyria to the north and Egypt to the south.
Unlike any other prophet, Isaiah receives his prophetic call in a vision during Temple worship. Jews believed that God "dwelled" on earth (i.e., the place they knew God was present) in the Temple. They also believed that the Temple itself was a mirrored image of God's heavenly court. In Isaiah's vision, the Temple does indeed become God's Holy court.
As an observer, Isaiah does not see God directly, only the train of his garment (i.e, his glory).  Seraphim have six wings: two to hover with, two to cover their faces (so they would not see God's glory and die), and two to cover their feet (i.e, their genital area for modestly sake). The angels cry out their worship; their words have been incorporated into our "Hosanna" at Mass. In response to their words, the doors shook (an earthquake?) and smoke filled the Temple; for the people of the time, an earthquake and smoke were both signs of divine presence. [2-5]
In the presence of God, Isaiah was anxious for his life because, in the face of such divine power, he could die. He was one of a sinful people who spoke with "unclean lips" (i.e., those who said one thing and did another). But his "sin" is removed by the touch of a burning coal to his lips; now Isaiah would only speak the truth and be singlehearted. With this sin removed he could speak in the holy court. [6-7]
God calls out for a prophet. Here, Isaiah's anxiousness turns to willingness. The "clean" Isaiah becomes God's prophet. 
Isaiah was a witness to God's power, first as an observer, then as a prophet. We are called to be God's witnesses, by what we see in our world and by what we do in our world. Seeing God's power requires prayer; doing God's will requires honesty and purpose.
How has God called you to be his witness? His prophet?