What does Mark 1:21 mean?

Mark 1:21 begins the third section of Mark, which outlines Jesus' ministry in Galilee (Mark 1:21—6:29). Jesus and His four newly-picked disciples—Andrew, Simon Peter, James, and John—are in Capernaum, where Simon Peter lives. On the Sabbath, Jesus goes into the synagogue to teach.

The Greek term sunagoge means "bringing together, gathering," and refers to the structure where such a gathering occurs. This center of teaching, in the Jewish faith, is called a "Synagogue." Some Jewish scholars believe synagogues existed from the time of Moses, though they most likely started during the Babylonian captivity when the Israelites were scattered all over the region and the temple was destroyed. Jews met in synagogues on the sabbath and feast days, to pray and listen to both the reading of Scriptures and commentary on the text. It was in a synagogue that Jesus announced His ministry in His hometown of Nazareth (Luke 4:16–21) and where Paul often started preaching the gospel (Acts 9:20; 17:17). A later version of the synagogue in Capernaum still stands mostly intact.

As the temple held the Ark of the Covenant, every synagogue features an "ark," or cabinet, containing the Torah. This is a set of hand-written scrolls of the Jewish Bible, corresponding to the Old Testament. It is still common practice for a non-priest / non-Levite to read from the Torah. After Jesus read, He would have sat down to teach.

Mark's focus in this verse emphasizes several factors that frustrate critics of the Bible. He provides details regarding location, and the city of Capernaum was well known. The day, place, and Jesus' actions are clear: He is teaching in the synagogue on a Saturday. There were many eyewitnesses, meaning Mark could not have easily fabricated this account. Doing so would have left too many ways to dismiss his story. The time, location, and witnesses could easily be investigated to verify whether the story was accurate. This makes it difficult to write off Mark's account as fictional.

However, Mark appears to write based on someone's eyewitness account—likely Peter who would have been in the room.