Acts 9 explains who sent him, "But the Lord said unto him, Go thy way: for he [Paul] is a chosen vessel unto me, to bear my name before the Gentiles and kings, and the children of Israel:" Acts 9:15 and in Gal 1:1 the apostle Paul makes clear where his authority comes from, "Paul, an apostle (not from men, neither through man, but through Jesus Christ, and God the Father, who raised him from the dead),"
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I find the events Paul experienced rather interesting. Prior to his conversion he went to an earthly High Priest and received some letters that contained an authority from this world. Upon meeting the heavenly High Priest there are new letters, they use the name of the heavenly authority, Yeshua. MATTHEW 28:18