What does it mean that the second beast exercises all the power of the first beast?
It means the second beast, the false prophet or religious leader, operates in the first beast's authority and service, working hand in glove with the Antichrist. Empowered by Satan, he uses religious deception and miracles to cause humanity to worship the first beast, the political leader. He is the counterfeit Holy Spirit directing worship to the counterfeit Son.
Les teaches that the second beast is the false prophet, a powerful religious leader under the inspiration of Satan. He exercises all the power of the first beast before him because the two are working hand in glove. As the counterfeit of the Holy Spirit, this religious leader causes the mass of humanity to worship the first beast, the political Antichrist, who is the counterfeit of God the Son. He performs great wonders, even making fire come down from heaven, and deceives those who dwell on the earth by means of his miracles, much as the genuine Holy Spirit draws people to God the Son.
And he exerciseth all the power of the first beast before him, and causeth the earth and them which dwell therein to worship the first beast, whose deadly wound was healed.