I, even I, am the Lord, and there is no savior besides Me. It is I who have declared and saved and proclaimed, and there was no strange god among you; so you are My witnesses, declares the Lord, and I am God. Even from eternity I am He, and there is none who can deliver out of My hand; I act and who can reverse it? (Isaiah 43:11-13)

But you will receive power when the Holy Spirit has come upon you; and you shall be My witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth. (Acts 1:8)

“It is interesting that Jesus spoke similar words to his disciples at his ascension (Acts 1:8). Like it or not, they were witnesses of who this man was in his life, death, and resurrection. Not until they were filled with the Holy Spirit (cf. Isa. 44:3) did they embrace this identity, but embrace it they did, as is evidenced especially in the prologue of 1 John (1:1–4), where the incontrovertible language of firsthand experience recurs again and again. Given that experience and its rootage by Jesus in the sweeping claims of Isa. 43, it is not surprising that the first Christian creed came to be “Jesus is Lord,” and that he should be understood to be the only Savior (cf. Isa. 43:11). Jesus himself taught the disciples to see that what was implicit in those words of Isaiah could find its fullest meaning only in him and in his work. (Oswalt, John N. The Book of Isaiah, Chapters 40–66 (the New International Commentary on the Old Testament). Eerdmans, 1998, 149.)