"Incline your ear, and come unto me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David. Behold, I have given him for a witness to the people, a leader and commander to the people. Behold, thou shalt call a nation that thou knowest not, and nations that knew not thee shall run unto thee because of the LORD thy God, and for the Holy One of Israel; for he hath glorified thee." (Isaiah 55:3-5)
In mercy, God continues to plead with the sinner. If a man is willing to lay aside his pride and run to God, the Lord will permanently deal with his sin problem; and that man's soul will find everlasting life. "He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life (John 5:24)." God reminds Israel of His covenant with David. Through David, God promised to bring the Messiah Who would deliver not only the Jews but all who put faith in Him. David was given by God as both a witness and a leader. Through his life and his writings, David witnessed of the coming Messiah and did much in leading the people of Israel toward their Redeemer. The mercies God promised to David are fulfilled in the work of Jesus Christ. These mercies cannot be reversed.
In the last verse, the words appear to be directed at Israel. In the millennial kingdom, the nations of the earth will hasten toward Israel. Such will be the case not because of self-righteous Judaism but because of the righteousness of Jesus Christ Who is Holy One of Israel. His physical throne will be found in the glorified land of the Jews, and all nations will serve the Messiah.