A witness is someone who proclaims a truth about someone else, be it good or bad. God has His witnesses, but the world at large does not recognize them as being his witnesses. They are the descendants of the lost tribes of Israel. They are the people that proclaim the Bible to be the revelation of God and use it to witness to the world that Jesus Christ is God.
The 44th chapter of the book of Isaiah is addressed to the people of Israel. “Yet now hear, O Jacob my servant; and Israel, whom I have chosen:” Isaiah 44:1. The next few verses are about promised blessings on Israel, and then in verse 6: “Thus saith the Lord the King of Israel, and his redeemer the Lord of hosts; I am the first, and I am the last; and beside me there is no God. And who, as I, shall call, and shall declare it, and set it in order for me, since I appointed the ancient people? and the things that are coming, and shall come, let them shew unto them. Fear ye not, neither be afraid: have not I told thee from that time, and have declared it? ye are even my witnesses. Is there a God beside me? yea, there is no God; I know not any.” Isaiah 44: 6-8. God says he is the First and the Last. He reiterates this in Isaiah 48:12, “Hearken unto me, O Jacob and Israel, my called; I am he; I am the first, I also am the last.” Centuries later the apostle John saw Jesus Christ enthroned in glory. “And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me, Fear not; I am the first and the last: I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.” Revelation 1: 17-18. Here is the same God, enthroned in glory. He spoke to the prophet Isaiah and later to the apostle John. We are his witnesses, that he (Jesus Christ) is God.