The fifth article of faith:
We believe that a man must be called of God, by prophecy, and by the laying on of hands by those who are in authority, to preach the Gospel and administer in the ordinances thereof.
This statement centers around our belief in a required priesthood authority.
We read in Hebrews 5:4, And no man taketh this honour unto himself, but he that is called of God, as was Aaron.
Ordinances such as baptism and receiving the gift of the Holy Ghost must be done under the correct authority. A man must be called of God, meaning that he must be given the authority from another that possesses it. This is done by the laying on of hands, something that is mentioned numerous times in the Bible.
Christ gave the priesthood authority to His apostles.
Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases. (Luke 9:1)
Talking specifically to Peter, Christ speaks of the priesthood authority as keys,
And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven. (Matthew 16:19)
The priesthood holds authority over the things pertaining to the earth as well as the heavens. Understanding this will help one to understand why Mormons are sealed in the temple for time and all eternity, and also why we do proxy baptisms for those no longer living on the earth.
In a nutshell, the priesthood is God’s power and authority. In these last days, He has reached out in love and restored it. Given to man, it allows man the right to act in His name. It is necessary to operate Jesus Christ’s church, to preach the gospel with the utmost power and spirit, and to perform saving ordinances such as baptism.