They called (ekaloun). Inchoative imperfect began to call.
Barnabas, Jupiter (ton Barnaban Dia). Because Barnabas was the older and the more imposing in appearance. Paul admits that he was not impressive in looks (2Co 10:10).
And Paul, Mercury (ton de Paulon Hermēn). Mercury (Hermēs) was the messenger of the gods, and the spokesman of Zeus. Hermēs was of beautiful appearance and eloquent in speech, the inventor of speech in legend. Our word hermeneutics or science of interpretation comes from this word (Heb 7:2; Joh 1:38).
Because he was the chief speaker (epeidē autos ēn ho hēgoumenos tou logou). Paul was clearly “the leader of the talk.” So it seemed a clear case to the natives. If preachers always knew what people really think of them! Whether Paul was alluding to his experience in Lystra or not in Gal 4:14, certainly they did receive him as an angel of God, as if “Mercury” in reality.