 They know this guy. They watched him grow up. He probably seemed like a well-behaved kid to them, but it's just hard to believe he's a mighty prophet when you know him. So they don't believe. And it seems at first glance that the sole reason they don't believe is simply because they know him. But there's a little bit more to it. They insult Jesus when they call him the son of Mary. Now this sounds fine to us. Jesus is the son of Mary. But it was typical in the ancient world