The whole town gathered at the door, 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 1:33-34
When Jesus healed people of demonic possession, he didn't allow the demons to speak and say who he was. Other times, like when talking to the demon possessed man, he spoke directly to demons, found out their name and carried out their request by putting them into a herd of pigs.
While Jesus communicated with demons, he didn't want them to be his primary spokesmen. Demons are not the most reputable sources. At the time in his ministry, at the onset, Jesus wanted his miracles, works and words to proclaim his authority. Not a bunch of scary demons.
The best testimony of Jesus' authority and identity is a saved and transformed life.
Not a demon.