An Apostle is a special witness of Jesus Christ. It is the term that the Savior Himself gave to the Twelve whom He chose and ordained to be His closest disciples and assistants during His earthly ministry. “...He called unto him his disciples: and of them he chose twelve, whom also he named apostles” (Luke 6:13). The late President Gordon B. Hinckley, a past president of [http:comeuntochrist.org The Church of Jesus Christ of Latter-day Saints]—sometimes inadvertently called the Mormon Church—said:
- The word apostle, in its origin, literally means “one sent forth.” If that definition were stated to say “one sent forth with certain authority and responsibility,” it would properly describe the calling as it was given at the time our Lord walked the earth, and as it has been given in our time. 
The Savior sent His Apostles to represent Him and to minister for Him following His ascension into heaven. Both anciently as well in Christ’s modern Church, Apostles testify of the divinity and resurrection from the dead of the Savior, Jesus Christ. (See Acts 1:22 and Doctrine & Covenants 107:23).
Apostles and Prophets are the Foundation of Christ’s Church
The scriptures teach that prophets and apostles perform essential roles in Christ’s Church. Paul wrote to the Ephesians that the Church is “built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief cornerstone” (Ephesians 2:20). The Savior taught Joseph Smith, the first prophet and president of the Church of Jesus Christ in modern days, the duties of apostles through revelations found in the Doctrine & Covenants—a book of scripture containing modern revelations.
The Twelve shall be my disciples, and they shall take upon them my name, and the Twelve are they who shall desire to take upon them my name with full purpose of heart. And if they desire to take upon them my name with full purpose of heart, they are called to go into all the world to preach my gospel unto every creature. And they are they who are ordained of me to baptize in my name, according to that which is written. (Doctrine & Covenants 18:27-29).
Further, the Twelve Apostles are to ordain, preach the gospel, and testify of Jesus Christ. They are commanded to walk uprightly before the Lord and not commit sins (Doctrine & Covenants 18:30-36).
Only the Prophets and Apostles Hold the Keys of the Kingdom of God
The Savior taught that His house “is a house of order ... and not a house of confusion” (Doctrine & Covenants 132:8). This means that His Church is organized and set up according to the laws and ordinances of His gospel. The First Presidency—which is the prophet and his two counselors—and the Quorum of the Twelve Apostles bear the priesthood keys of the Lord’s ministry (Doctrine & Covenants 27:12). The prophet and his counselors are also apostles of Jesus Christ. The priesthood is the power that God gives to man to act in all things for the salvation of His children. The keys of the priesthood are the power given to man to direct, control and govern His priesthood on earth. Thus, the keys of His ministry include the power to direct His Church. Only one man holds all of the keys to the Lord’s kingdom on earth—His prophet. When a prophet of God dies, the Quorum of the Twelve Apostles becomes the presiding body of the Church until a new president is called and ordained. As the Lord’s house is one of order, the new president of the Church is always the senior apostle of the Lord.
Thus, the prophets and apostles are the foundation upon which the Church is built, because they hold the keys—the power given to them from Christ Himself, the head of the Church, through the proper priesthood channels—to direct, govern and control the affairs of the Church.
The Keys to Christ’s Church are Restored Through His Apostles
This organization existed in Christ’s ancient Church. After the death of the Savior and His apostles, the fullness of His gospel was lost because those who held the keys had died. This dark period of time, when Christ’s true Church was not on the earth, is called the Apostasy. The late Boyd K. Packer, a former member of the Quorum of the Twelve Apostles, said:
- The most precious thing lost in the Apostasy was the authority held by the Twelve—the priesthood keys. For the Church to be His Church, there must be a Quorum of the Twelve who hold the keys and confer them on others. 
The Articles of Faith are thirteen statements outlining the basic beliefs and doctrines of The Church of Jesus Christ. The Sixth Article of Faith states: “We believe in the same organization that existed in the Primitive Church, namely, apostles, prophets, pastors, teachers, evangelists, and so forth.” For the same organization to exist, the keys of the kingdom also had to be restored.
The keys of the kingdom of God could not be conferred upon the Prophet Joseph Smith by any persons other than those who still held them. Thus, as the late Apostle Elder James E. Faust explains, only heavenly messengers could restore these keys. Elder Faust said:
- Peter, James, and John conferred the keys of the kingdom of God upon the Prophet Joseph and ordained him to be an Apostle and a special witness of the name of the Savior and to bear the keys of his ministry (Doctrine & Covenants 27:12-13). Keys that pertain to the gathering of Israel, the dispensation of Abraham, and, indispensably, the keys of sealing were conferred upon the Prophet by Moses, Elias, and Elijah in 1836. (See Doctrine & Covenants 110.)
- Prior to [his] martyrdom, no doubt with a sense of foreboding, the Prophet Joseph prepared for his death. President Joseph Fielding Smith states: “The Prophet declared that he knew not why, but the Lord commanded him to endow the Twelve with these keys and priesthood, and after it was done, he rejoiced very much, saying in substance, ‘Now, if they kill me, you have all the keys and all the ordinances and you can confer them upon others, and the powers of Satan will not be able to tear down the kingdom as fast as you will be able to build it up, and upon your shoulders will the responsibility of leading this people rest.’” [Joseph Fielding Smith, Doctrines of Salvation, comp. Bruce R. McConkie, 3 vols. (Salt Lake City: Bookcraft, 1954–56), 1:259.] 
Apostles of the Lord hold that office until they die. Through the living prophet, apostolic keys are conferred upon the next apostle. When a prophet dies, each member of the Quorum of the Twelve Apostles confers his keys upon the new prophet. Thus, the prophet is the only man who holds all the keys to the kingdom of God. And the priesthood line of succession remains unbroken.
Apostles are Prophets, Seers and Revelators
Prophets and apostles are the Lord’s mouthpieces, sounding not only a witness to the Savior, Jesus Christ, but also a warning cry for repentance unto the peoples of the world. The apostle Paul told the Ephesians that Christ ordained apostles to teach “that we henceforth be no more children, tossed to and fro and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive” (Ephesians 4:14). Elder Jeffrey R. Holland, an apostle of Jesus Christ, said:
- Thus the apostolic and prophetic foundation of the Church was to bless in all times, but especially in times of adversity or danger, times when we might feel like children, confused or disoriented, perhaps a little fearful, times in which the devious hand of men or the maliciousness of the devil would attempt to unsettle or mislead. Against such times as come in our modern day, the First Presidency and Quorum of the Twelve are commissioned by God and sustained by you as prophets, seers, and revelators, with the President of the Church sustained as the prophet, seer, and revelator, the senior Apostle, and as such the only man authorized to exercise all of the revelatory and administrative keys for the Church. 
Elder Faust said:
- I strongly counsel all who have membership in this church to follow the teachings and counsel of those who now have the keys as prophets, seers, and revelators. They are the ones who will inspire us to deal with the vicissitudes of our time. I plead with all not to try to selectively invoke gospel principles or scripture to wrongly justify spiritual disobedience, or to separate themselves from the responsibilities of covenants and ordinances contrary to the counsel of those who have the prophetic voice in the Church. ... Great temporal and spiritual strength flows from following those who have the keys of the kingdom of God in our time.