Joseph Smith - The Mission of John the Baptist

Regarding the priesthood, the Lord said -

"...all they who receive this priesthood receive me, saith the Lord;
36 For he that receiveth my servants receiveth me..." (Doctrine and Covenants Section 84:35 - 36)

The Savior is teaching that receiving His priesthood is synonomous to receiving His servants who are our priesthood leaders. He explained -

"...whether by mine own voice or by the voice of my servants, it is the same." (Doctrine and Covenants Section 1:38)

The Prophet Joseph Smith taught -

“Some say the kingdom of God was not set up on the earth until the day of Pentecost, and that John [the Baptist] did not preach the baptism of repentance for the remission of sins; but I say, in the name of the Lord, that the kingdom of God was set up on the earth from the days of Adam to the present time. Whenever there has been a righteous man on earth unto whom God revealed His word and gave power and authority to administer in His name, and where there is a priest of God—a minister who has power and authority from God to administer in the ordinances of the gospel and officiate in the priesthood of God, there is the kingdom of God; and, in consequence of rejecting the Gospel of Jesus Christ and the Prophets whom God hath sent, the judgments of God have rested upon people, cities, and nations, in various ages of the world…"

The Prophet explained the relationship between the priesthood, God's servants, and the kingdom of God -

“But, says one, the kingdom of God could not be set up in the days of John, for John said the kingdom was at hand. But I would ask if it could be any nearer to them than to be in the hands of John. The people need not wait for the days of Pentecost to find the kingdom of God, for John had it with him, and he came forth from the wilderness crying out, ‘Repent ye, for the kingdom of heaven is nigh at hand’ [Matthew 3:2], as much as to say, ‘Out here I have got the kingdom of God and I am coming after you; I have got the kingdom of God, and you can get it, and I am coming after you; and if you don’t receive it, you will be damned;’ and the scriptures represent that all Jerusalem went out unto John’s baptism [see Matthew 3:5–6]. There was a legal administrator, and those that were baptized were subjects for a king; and also the laws and oracles of God were there; therefore the kingdom of God was there; for no man could have better authority to administer than John; and our Savior submitted to that authority Himself, by being baptized by John; therefore the kingdom of God was set up on the earth, even in the days of John. …"

He continued -

“Whenever men can find out the will of God and find an administrator legally authorized from God, there is the kingdom of God; but where these are not, the kingdom of God is not. All the ordinances, systems, and administrations on the earth are of no use to the children of men, unless they are ordained and authorized of God; for nothing will save a man but a legal administrator; for none others will be acknowledged either by God or angels.”

“John [the Baptist] held the Aaronic Priesthood, and was a legal administrator, and the forerunner of Christ, and came to prepare the way before him. … John was a priest after the order of Aaron before Christ. …
“The keys of the Aaronic Priesthood were committed unto him, and he was as the voice of one crying in the wilderness, saying, ‘Prepare ye the way of the Lord and make his paths straight.’ [Matthew 3:3.] …"

This being true, the kingdom of God was once again established on the earth on May 15, 1829 on the banks fo the Susquehanna River when -

"...The ordination was done by the hands of an angel, who announced himself as John, the same that is called John the Baptist in the New Testament. The angel explained that he was acting under the direction of Peter, James, and John, the ancient apostles, who held the keys of the higher priesthood, which was called the Priesthood of Melchizedek. The promise was given to Joseph and Oliver that in due time the Priesthood of Melchizedek would be conferred upon them..." (Doctrine and Covenants Section 13:Heading)

Saying -

"UPON you my fellow servants, in the name of Messiah I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth, until the sons of Levi do offer again an offering unto the Lord in righteousness." (Doctrine and Covenants Section 13:1)