Where does the Apostle Paul get the authority for this ruling?
1CO 5:1 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife. [2] And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you. [3] For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed, [4] In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ, [5] To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.