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Question: Why is priesthood authority transferred by the "laying on of hands"?
The apostles themselves called others to assist them and gave them authority by laying on of hands. These men did not select themselves, and they did not begin to act as assistants to the Apostles until they were given authority:
1 AND in those days, when the number of the disciples was multiplied, there arose a murmuring of the Grecians against the Hebrews, because their widows were neglected in the daily ministration.
2 Then the twelve called the multitude of the disciples unto them, and said, It is not reason that we should leave the word of God, and serve tables.
3 Wherefore, brethren, look ye out among you seven men of honest report, full of the Holy Ghost and wisdom, whom we may appoint over this business.
4 But we will give ourselves continually to prayer, and to the ministry of the word.
5 And the saying pleased the whole multitude: and they chose Stephen, a man full of faith and of the Holy Ghost, and Philip, and Prochorus, and Nicanor, and Timon, and Parmenas, and Nicolas a proselyte of Antioch:
6 Whom they set before the apostles: and when they had prayed, they laid their hands on them. (Acts 6:1-6).
Paul and Barnabas were chosen in the same manner: those with authority received revelation from God (via the Holy Ghost) about who should be chosen, and then gave them authority by the laying on of hands:
2 As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them.
3 And when they had fasted and prayed, and laid their hands on them, they sent them away. (Acts 13:2-3).
The ability to give the Holy Ghost by the laying on of hands was clearly understood to be a power from God that had to be conferred:
18 And when Simon saw that through laying on of the apostles' hands the Holy Ghost was given, he offered them money,
19 Saying, Give me also this power, that on whomsoever I lay hands, he may receive the Holy Ghost.
20 But Peter said unto him, Thy money perish with thee, because thou hast thought that the gift of God may be purchased with money.
21 Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God.
22 Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.
23 For I perceive that thou art in the gall of bitterness, and in the bond of iniquity. (Acts 8:18-23).
Simon the magician tried to buy the power, and was rebuked by Peter: notice, though, that Simon understands very clearly that he cannot do what the apostles are doing without the authority: he simply tries to get the authority in the wrong way.