Why were the demons commanded not to speak because they knew who Jesus was (Mark 1:34)?
In Mark's Gospel the only ones who seem to know that Jesus is the Son of God are the readers, who are told from the beginning, and the demons. But Jesus commands the demons to be silent. Following healings, Jesus also frequently commands people to tell no one what has happened (1:40-45). Sometimes they obey; sometimes they don't. An important example of this is when Peter confesses Jesus to be the Messiah and is sternly ordered not to tell anyone about him (8:27-30).
This phenomenon, often called the "Messianic secret," has puzzled generations. The most likely reason Jesus commands silence is that until his death and resurrection any proclamation of him as Messiah would be misunderstood. His true messianic work wouldn't be his miracles, healings or exorcisms but his death on the cross.
He was a servant Messiah, who in weakness and vulnerability dies an unmessianic death, one thought impossible if he were the Son of God. But this is precisely God's will. To proclaim him Son of God and Messiah for his deeds of power would send the opposite message of how God intends to save people through Jesus.
In Mark the disciples are as blind and disobedient as Jesus' enemies. If they are to be useful, they will have to be healed and transformed, which will come only after the Resurrection. Jesus will fulfill all of God's promises but only in God's way and time.
By using his name and showing they know who Jesus is, the demons aren't making a statement of faith. They hope to have power over Jesus' ability to cast them out. It didn't work since Jesus silenced them. Also, Peter's confession that Jesus is the Messiah isn't only premature but was understood by him in such a way that, if Jesus followed it, it would have been a satanic temptation.
The ultimate irony in Mark is that when Easter finally comes, the women who discover the empty tomb flee in terror--and say nothing (16:1-8).
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