Therefore, if anyone is in Christ, he is a new creation; the old has gone, the new has come (2 Cor 5:17)!

If you are in Christ, this is the reality in which you live. The reality you used to live has become old and disappeared, and a new reality has come. When the reality you live in changes, the way you live changes. When you are a single, you live as a single. But when you get married, which is a new reality, you start living as a husband or a wife. Then, when a baby is born, which is another new reality, you begin to live as a father or a mother. In the same way, if you are in Christ by having been baptized into Christ and his death, you are a new creation—the fulfillment of God’s word, and live in a new reality. You cannot live in the old reality you used to live in.

Forget the former things; do not dwell on the past. See, I am doing a new thing! Now it springs up; do you not perceive it? I am making a way in the desert and streams in the wasteland (Isa 43:18-19).

Here, “the former things” and “the past” mean Satan’s work of death and destruction that happened in the past. And “a new thing” means God’s work of creation and life. The work of destruction has gone, and the reality of death has disappeared. To “forget” and “not dwell on” means not to accept as your reality. We must not accept the old reality of destruction and death because what was destroyed was created back, and death has been swallowed in victory. What was lost was restored to us, and the word of life was fulfilled in us. We are the witnesses of that.

Lead out those who have eyes but are blind, who have ears but are deaf. All the nations gather together and the peoples assemble. Which of them foretold this and proclaimed to us the former things? Let them bring in their witnesses to prove they were right, so that others may hear and say, "It is true."

Here, “the former things” are God’s work of creation we have experienced in the past. We should know that all such work was done as his word was fulfilled. Nonbelievers have not experienced or seen such work, nor believe that God has the power to fulfill his word. Therefore, God wants us to go out and testify before them as his witnesses.

"You are my witnesses," declares the Lord, "and my servant whom I have chosen, so that you may know and believe me and understand that I am he (v. 10).”

“I am he” means “I am the one who fulfills my word”: He is Yahweh, the Lord. There are three things we need to do: to know, to believe, and to understand. First, we need to know what he says, the truth—his will and plan. Second, we need to believe that God has the power to do what he says so that what he says will be fulfilled. Third, after it was fulfilled, we need to understand that what occurred to us was the fulfillment of his word.

I, even I, am the Lord (Yahweh), and apart from me there is no savior. I have revealed and saved and proclaimed-- I, and not some foreign god among you. You are my witnesses," declares the Lord, (vv. 11-12)…

Note the strong emphasis on “I” at the beginning of the sentence. He is the only God, and there is no other God. He is Yahweh, the God who faithfully fulfills his word. Note also the three verbs used here: to reveal, to save and to proclaim (lit. to let someone hear). He revealed his plan and will to us with his word, and saved us by fulfilling it, and proclaimed to us, or let us hear and see that it was by the fulfillment of his word that we were saved. We are the witnesses of that.