A.T. Robertson Commentary Acts 26:12

"Whereupon as I journeyed to Damascus with the authority and commission of the chief priests," — Acts 26:12 (ASV)

Whereupon (εν οις). "In which things" (affairs of persecution), "on which errand." Cf. 24:18. Paul made them leave Palestine (11:19) and followed them beyond it (9:2).

With the authority and commission (μετ' εξουσιας κα επιτροπης). Not merely "authority" (εξουσια), but express appointment (επιτροπη, old word, but here only in N.T., derived from επιτροπος, steward, and that from επιτρεπω, to turn over to, to commit).