Acts 14:12 describes how the people in the city of Lystra reacted to Paul and Barnabas after witnessing a miracle: they called Barnabas "Jupiter" (the Roman god Zeus) and Paul "Mercurius" (the Roman god Hermes), because Paul was the main speaker. The crowd believed the two men were gods visiting them in human form.
In this scene, Paul and Barnabas are in Lystra, a city in the Roman province of Galatia, speaking to a mostly non-Jewish audience. After Paul heals a man who had never walked, the local people interpret the miracle through their own religious beliefs, identifying Barnabas with Zeus (the chief god) and Paul with Hermes (the messenger god), since Paul did most of the talking. This reaction sets the stage for the confusion and challenges that follow in the story.