Possible meanings are 1) Luke expected the readers to know which city he was writing about. Alternate translation: "the main city in Samaria" or 2) Luke did not expect his readers to know which city he was writing about. Alternate translation: "a city in Samaria"
The title "Christ" refers to Jesus, the Messiah. Alternate translation: "told them about Jesus Christ" or "told them about Jesus the Messiah" (See: [[rc://en/ta/man/translate/figs-metonymy]])