And they called Barnabas, Jupiter - Jupiter was the most powerful of all the gods of the ancients. He was represented as the son of Saturn and Ops, and was educated in a cave on Mount Ida, in the island of Crete. The worship of Jupiter was almost universal. He was the Aremon of Africa, the Belus of Babylon, the Osiris of Egypt. His common appellation was, The Father of gods and men. He was usually represented as sitting upon a golden or an ivory throne, holding in one hand a thunderbolt, and in the other a scepter of cypress. His power was supposed to extend over other gods; and everything was subservient to his will except the Fates. There is the most abundant proof that he was worshipped in the region of Lycaonia and throughout Asia Minor. There was, besides, a fable among the inhabitants of Lycaonia that Jupiter and Mercury had once visited that place, and had been received by Philemon. The whole fable is related by Ovid, “Metam.,” 8, 611, etc.
And Paul, Mercurius - Mercury, called by the Greeks Hermes, was a celebrated god of antiquity. No less than five of this name are mentioned by Cicero. The most celebrated was the son of Jupiter and Maia. He was the messenger of the gods, and of Jupiter in particular; he was the patron of travelers and shepherds; he conducted the souls of the dead into the infernal regions; he presided over orators, and declaimers, and merchants; and he was also the god of thieves, pickpockets, and all dishonest persons. He was regarded as the god of eloquence; and as light, rapid, and quick in his movements. The conjecture of Chrysostom is, that Barnabas was a large, athletic man, and was hence taken for Jupiter; and that Paul was small in his person, and was hence supposed to be Mercury.
Because he was the chief speaker - The office of Mercury was to deliver the messages of the gods; and as Paul only had been discoursing, he was supposed to be Mercury.