Mk 8:11-13
No signs: For Mark, authentic faith in Jesus is not based on signs. He places the short narrative of the Pharisees’ demand for a sign immediately after the narrative of the healing of the deaf man and the feeding of the four thousand. It would be easy for the reader to conclude that these signs (healing, multiplication of loaves and fishes) are sufficient reasons to believe in Jesus. For Mark, however, miracles or signs do not prove anything. A person believes in Jesus because Jesus is like them—powerlessly human!
In refusing to give the reader any sign, Mark is being faithful to the first commandment—God has no image; he is not an idol. To make anyone an idol or a sign is to fall into the temptation of idolatry. Mark does not want to turn Jesus into an idol. He portrays Jesus as one who exactly resembles God who cannot be imaged.
For Mark, Jesus is like God in the sense that the most that anyone can say about God is nothing, and when nothing has been said, all that can be said has been spoken! Therefore, once a lot has been said about Jesus, the author must conclude that whatever has been said must be unsaid (nothing), otherwise, he would have created an idol, a sign.
A person either believes or does not believe. The furthering of this Gospel message is left to the reader who is cautioned to put no stock in signs.