These "witnesses" must meet some very specific criteria given in Isaiah chapter 43. Verse 3 says they were:
1) Delivered from the land of Egypt.
2) They offered animal sacrifices to the Lord in verse 23.
3) The law covenant that they entered into they violated and trangressed in verses 24-25.
4) For the Lord's sake He forgives them in verse 25.
5) To these same witnesses God says in verses 5-6, "I will bring thy seed from the east, and gather thee from the west; I will say to the north, Give up; and to the south, Keep not back: bring my sons from far, and my daughters from the ends of the earth."
This narrow set of criteria that God addresses in chapter 43 is required to be true of any who claim to be witnesses of Jehovah. It certainly would not be true of Christians for they have not been brought out of literal Egypt neither have they offered animal sacrifices because Jesus the Lamb of God has become their "once for all" eternal sacrifice. But it would be true of Israel as a nation.
These people were brought out of Egypt by God in Exodus chapters 11-14. They were given specific instructions for daily, monthly, yearly and other applicable sacrifices in the book of Leviticus. Israel entered into a very specific covenant of laws and ordinances beginning in Exodus chapter 19:8 saying, "All that the LORD hath spoken we will do." But before Israel left that campsite by the mountain in Sinai they had violated this covenant and also sought forgiveness as recorded in chapter 32:1-35. [see: Judges 2:19-27]
Later after entering into the land promised to Abraham in Genesis 17:1-8, the nation of Israel was scattered (Ezekiel 39:23) under several waves of invasions. First the northern tribes were taken captive and dispersed by the Assyrians in 733 B.C. (2 Kings 17:5-6, 22-23). The southern kingdom of Judah was carried away into Babylonian Captivity in about four stages of expulsions (2 Chronicles 36; Daniel 1; 2 Kings 24). God said this Babylonian captivity would last seventy years in Jeremiah 25:11 & Daniel 9:2. And even though some of Israel returned again under Cyrus and Darius only a remnant was restored to the land, not all of the exiled (the whole nation). (Isaiah 10:20-25; Jeremiah 23:3-4). Again Israel was conquered by the Greeks with Alexander the Great and more importantly later by the Romans. They were scattered again in 70AD by the Roman ruler Titus throughout the Roman empire and dispersed even more beyond that. The promises and prophecies in Isaiah 43:5-6; 27:12; Jeremiah 32:37-41; Ezekiel 11:16-17; 37:15-28 and elsewhere remain to be fulfilled.
It is evident that only Israel, who was taken out of Egypt, given a covenant with animal sacrifices, dispersed to other nations for their gross disobedience and gathered again into their own land since the early 20th century, fulfills the 5 specific items given in Isaiah chapter 43. Only they are and can be witnesses of the LORD (Jehovah) for "Ye are my witnesses, saith the Lord."