"For he was baptized while he was yet in his childhood, and was ordained by the angel of God at the time he was eight says old unto this power, to overthrow the kingdom of the Jews, and to make straight the way of the Lord before the face of his people, to prepare them for the coming of the Lord, in whose hand is given all power" (D&C 84:28). This verse is about John the Baptist. He was a great prophet - someone who prepared Israel for the coming of the Messiah. An interesting part of this verse is how John was ordained in part, "to overthrow the kingdom of the Jews." How did John overthrow, or at least pave the way for the kingdom of the Jews to be overthrown?
Joseph Smith listed three reasons John the Baptist was not only a great prophet but also one who would overthrow the kingdom of the Jews: "First. [John] was entrusted with a divine mission of preparing the way before the face of the Lord. Whoever had such a trust committed to him before or since? No man.
"Secondly. He was entrusted with the important mission, and it was required at his hands, to baptize the Son of Man. ...
"Thirdly. John, at that time, was the only legal administrator in the affairs of the kingdom there was then on the earth, and holding the keys of power. The Jews had to obey his instructions or be damned, by their own law; and Christ Himself fulfilled all righteousness in becoming obedient to the law. ...The son of Zacharias wrested the keys, the kingdom, the power, the glory from the Jews, by the holy anointing and decree of heaven, and these three reasons constitute him the greatest prophet born of a woman" (Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith [Salt Lake City: Deseret Book Co., 1938], pp. 275–76).
John "wrested the keys, the kingdom, the power, the glory from the Jews, by the holy anointing and decree of heaven." By the time John lived, the scribes and pharisees had perverted and distorted the Judaic Law. Even the priests had largely gone astray. There were many good ones, such as Zacharias - John's father, but they did not rightly hold any priesthood keys. John was blessed when eight days old and told that he would later receive the Aaronic Priesthood as well as the keys of that priesthood. He did. When John received the keys, he had the authority to administer the temporal ordinances of the Lord, including baptism. He also held the authority to teach the gospel and hold the Israelites accountable unto God if they refused John's message.
John's whole mission was to prepare the way for the Savior. His light diminished once the Savior's arose. John did not seek fame nor did he feel threatened when Jesus' mission started. To the contrary, John understood his mission and fulfilled it admirably. He was one of God's elect, chosen before the world began to prepare the way of the Lord. There were few others like him. He gave His life - literally - for God's kingdom. He condemned sin and spoke boldly in the face of opposition. He unflinchingly faced Herod and Herodias when they sought and later, took his life.
Many might look at John's life and think his life was a failure, that he accomplished little of significance; that he certainly did not overthrow the kingdom of the Jews. They might say, "The kingdom of the Jews was not overthrown by John; the Jews were already under governance of the Romans and would shortly be destroyed and scattered with such finality that they would not have their own land again where the morning sun would reveal hill and plain for nearly two thousand years. John had nothing to do with their overthrow." But that is not what was meant in the Doctrine and Covenants. John did not overthrow the earthly kingdom of the Jews any more than the Savior did during His ministry. John overthrew the Jew's spiritual kingdom. Hepaved the way for the Savior to fulfill the law of Moses. John was "the greatest prophet born of woman."