John Calvin Commentary Mark 1:34

"And he healed many that were sick with divers diseases, and cast out many demons; and he suffered not the demons to speak, because they knew him." — Mark 1:34 (ASV)

He did not permit the devils to speak. There might be two reasons why he did not permit them: a general reason, because the time of the full revelation had not yet come; and a special reason, which we hinted at previously, that he refused to have, as heralds and witnesses of his divinity, those whose praise could have no other effect than to tarnish and damage his character. This latter reason is undoubtedly true: for he must have known, that the prince of death, and his agents, are in a state of irreconcilable enmity with the Author of eternal salvation and life.