He would not let the demons speak

“and Jesus healed many who had various diseases. He also drove out many demons, but He would not let the demons speak because they knew who He was.” Mark 1:34
Every time we see Jesus driving out demons, we read how He would not permit them to speak, or if they did, declaring Him the Son of God, He would tell them to be silent. See Mark 1:24-25. A similar instance might be found in Acts 16:17-18, where Paul was troubled by the slave girl calling attention to them.
There are a few things to note here. Firstly, the demons believe Jesus is the Son of God. They believe He has the power to judge them. They believe in hell. How many people do we know, who call themselves Christians, who don’t believe these things!
Secondly: Why would Jesus forbid them to speak? Although they are telling the truth, they are wanting to make a scene and draw attention to themselves. When Peter confessed Christ’s divinity (Matt. 16:16), Jesus told him, “Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by My Father in heaven” (Matt. 16:17).
The answer is this: Jesus wanted people to make up their own minds as to Who He was, without being influenced by a demon. Unfortunately, the same thing happens today. People get into Christianity because of some demonic weirdness that amazes them. But if you can get into Christianity through a demon, you can get out of it too. It is far better to come to know the Lord personally, and to make that your reason for believing.