I know who you are: the Holy One of God (Mark 1, 24)
Jesus and his disciples went as far as Capernaum, and as soon as the sabbath came he went to the synagogue and began to teach. And his teaching made a deep impression on them because, unlike the scribes, he taught them with authority. In their synagogue just then there was a man possessed by an unclean spirit and it shouted, ‘What do you want with us, Jesus of Nazareth? Have you come to destroy us? I know who you are: the Holy One of God.’ But Jesus said sharply, ‘Be quiet! Come out of him!’ And the unclean spirit threw the man into convulsions and with a loud cry went out of him. The people were so astonished that they started asking each other what it all meant. ‘Here is a teaching that is new’ they said ‘and with authority behind it: he gives orders even to unclean spirits and they obey him.’ And his reputation rapidly spread everywhere, through all the surrounding Galilean countryside.(Mark 1, 21-28)
The devils know very well who Jesus is, but because they have betrayed God their aim is to subvert the order of divine will. This is why Jesus commands them to be silent even if they are telling the truth. Whenever we commit a sin, we behave exactly like these devils: we do something that is not God's will and that distances us from Him.