MARK 1.23-24: And immediately there was in their synagogue a man with an unclean spirit. And he cried out, (24) “What have you to do with us, Jesus of Nazareth? Have you come to destroy us? I know who you are — the Holy One of God.” (ESV)
An unclean spirit is also referred to in the Bible as a “demon”, or an “evil spirit”. These unclean spirits were cast out of heaven with Satan at the time of Satan’s rebellion against God. Satan himself is a demon and he is the leader in his evil kingdom against God the Father, Jesus the Son, and the Holy Spirit. Satan and his demons are completely subordinate to the Triune God, and they only have the power God allows them to have.
Satan and his forces are not all-powerful, all-knowing, or anything of the sort, as they work in the world. But they are formidable. They are schemers and strategists and tacticians against all people everywhere. But they only have the authority against us we allow them to have. And whenever we resist them, opposing them properly by the power of Christ Jesus, they must flee.
In the verses above, the Holiness of Jesus so provoked and intimidated the unclean spirit that it cried out to Jesus in fear. It would have preferred to say nothing and thereby stay hidden. But it could not. It reflexively exposed itself out in the open.
In some of the gospel accounts when demons speak through people there can be confusion on our part regarding the singularity of a demon present, or a plurality of demons present in the event. In this case, when the demon speaks, it does so using a singular verb for itself, but it also uses the plural pronoun “us”. The demon is most likely referring to itself and the person it inhabited when speaking of “us”.
The demon, speaking by and through the man it was living in, acknowledged that they knew exactly who Jesus was. He was and is the “Holy One of God”, meaning that Jesus is the Messiah of God.