Mark 1:21–28 (ESV)
21 And they went into Capernaum, and immediately on the Sabbath he entered the synagogue and was teaching. 22 And they were astonished at his teaching, for he taught them as one who had authority, and not as the scribes. 23 And immediately there was in their synagogue a man with an unclean spirit. And he cried out, 24 “What have you to do with us, Jesus of Nazareth? Have you come to destroy us? I know who you are—the Holy One of God.” 25 But Jesus rebuked him, saying, “Be silent, and come out of him!” 26 And the unclean spirit, convulsing him and crying out with a loud voice, came out of him. 27 And they were all amazed, so that they questioned among themselves, saying, “What is this? A new teaching with authority! He commands even the unclean spirits, and they obey him.” 28 And at once his fame spread everywhere throughout all the surrounding region of Galilee.
The irony of this account which Mark is telling is that the demons even know who Jesus is but in the eyes of the other characters witnessing these events and perhaps even in the minds of Mark’s readers the identity of Jesus is still in question. This is a primary theme of the book that Mark will tease out throughout the first 8 chapters primarily, culminating in Peter’s declaration of Jesus as the Messiah in Mark 8:27-30.
Interestingly Jesus rebukes the demon for these remarks and commands it to be silent. To speculate as to his reasons at this point would be reading too much into the text yet many ideas have been offered. The best answer comes in verse 34 where Mark tells us that it was because they knew him. So it seems as if Jesus simply doesn’t want demons to declare his true identity.
However, without the gospel writers or Jesus himself specifically indicating why he demands silence, these must remain as mere speculation - meaning, don’t make too much out Jesus’ demand for silence, instead focus on the fact that even the demons know who he is. Yes! It is that obvious. Jesus is the Messiah, the Holy One of God.
At this point in the narrative we've seen God the Father indicate Jesus identity at his baptism and the demon indicate his identity. We are yet to have a person express the true identity of Jesus. So for the people surrounding Jesus at this time the jury is still out on who he is. Yet, he is super popular as people are coming from all over to see him. Jesus' popularity is a theme that we will continue to trace as we go through the story of Jesus in Mark.
Note the similarities between this encounter with the demon and the encounter in Mark 5:1-20. In that encounter we see the same themes of Jesus' authority and identity taking precedence again. However, at that point in the narrative we see more of a rejection from the people in the community.