Throughout history God has called prophets. Adam, Noah, Abraham, Moses, and many others. They were called of God and given the priesthood, which is power and authority given to man to act in God's name. They taught the people about Jesus Christ and what they needed to do to follow Him. Amos 3: 7 reads, "Surely the Lord God will do nothing, but he revealeth his secret unto his servants the prophets." Everyone has their ability to choose, some chose to follow their teachings and others chose not to. When the people followed the prophets, they were blessed. When the people chose to reject the prophets, God would withdraw His priesthood authority to teach and administer the ordinances of the gospel.
