In their synagogues.
He was casting out
καὶ ἦλθεν κηρύσσων εἰς τὰς συναγωγὰς αὐτῶν εἰς ὅλην τὴν Γαλιλαίαν καὶ τὰ δαιμόνια ἐκβάλλων.
This was another of Mark’s summary statements. Luke, chapter 4:44, has something similar, but Luke said that it was Judea and not Galilee. Besides, Luke did not mention anything about casting out demons. Matthew, chapter 4:23, is also somewhat similar. Matthew implied that Jesus went all over Galilee, as he was teaching in their synagogues. He said that Jesus was proclaiming the good news or the gospel about the kingdom, without saying whether it was the kingdom of God, the kingdom of heaven, or even an earthly kingdom. Here Mark said that Jesus went throughout the whole of Galilee (καὶ ἦλθεν…εἰς ὅλην τὴν Γαλιλαίαν), as he proclaimed or preached this unspecified message in their synagogues (κηρύσσων εἰς τὰς συναγωγὰς αὐτῶν). The synagogue was a new developing Jewish gathering place that might mean a group or assembly of Jewish people rather than a building, since some places may not have been able to afford a building. At the same time, Jesus was casting out demons (καὶ τὰ δαιμόνια ἐκβάλλων). Mark seemed very intent on emphasizing that Jesus was casting out demons along with his undefined preaching. He gave the impression that this took place all over Galilee without mentioning any particular place.