Why the command of silence in 8:30?

Jesus knew that Peter and the other disciples did not understand the significance of His message--yet. Certainly, Peter had seen that Jesus was the Christ, the Son of God. He had, at last, seen the answer to the question of Jesus' identity. What he did not yet understand was the nature of Jesus' ministry. In the coming days, Jesus would patiently unfold that to him (and the other disciples). But as yet, Peter's poor spiritual vision was only partly healed. Like the blind man of Bethsaida, Peter needed a 2nd touch from Jesus, in order to see properly. The first touch had enabled him to see Jesus' identity. The second would help Peter to see 'everything clearly' (Mark 8:25). This 'second touch' is described in the rest of Mark's gospel... from 8:31 to the end.

--Sinclair Ferguson, Let's Study Mark, page 130