"Then I heard the voice of the Lord saying, 'Whom shall I send? And who will go for us?' And I said, 'Here am I. Send me!'" NIV translation
Now that Isaiah's mouth has been purified, he can be used for God's purposes. When God asks, Isaiah answered the call to ministry. This same pattern occurs in believers today as they grow in Christ. God purifies us from sin, through the blood of Jesus Christ, and then he calls us to service. The service does not provide us with eternal life but it is just a natural expression of love and thanks when we are saved and freed from the trap of sin and death. As with Isaiah, God has a plan for each and every one of us and, when he calls, our answer should be the same: "Here am I. Send me!"
"He said, 'Go and tell this people: "Be ever hearing, but never understanding; be ever seeing, but never perceiving. Make the heart of this people calloused; make their ears dull and close their eyes. Otherwise they might see with their eyes, hear with their ears, understand with their hearts, and turn and be healed."'" NIV translation
At first reading this passage may seem like God does not want his people to turn to him but that is not the case. In the Septuagint, it is worded different and it is clear that God is telling Isaiah that the people are already the way that is described. God is explaining to Isaiah the condition of the people and preparing him for his work.
"Then I said, 'For how long, O Lord?' And he answered, 'Until the cities lie ruined and without inhabitant, until the houses are left deserted and the fields ruined and ravaged, until the Lord has sent everyone far away and the land is utterly forsaken.'" NIV translation
Isaiah asked God how long he was going to have to speak to a people that were not going to listen. God's answer wasn't exactly uplifting as he describes the exile of the people from the land. Sometimes, we may have the same type of experience as Christians proclaiming the gospel of Jesus Christ. There are people that may never listen and be saved but we still must continue to proclaim just as Isaiah did.
"'And though a tenth remains in the land, it will again be laid waste. But as the terebinth and oak leave stumps when they are cut down, so the holy seed will be the stump in the land.'" NIV translation
God is speaking to Isaiah here and gives a word of hope as he describes the remnant that will remain. He likens it to cutting down a tree and leaving a stump.