What does Mark 1:26 mean?

Although demon-possessed persons in the Bible often appear to be suffering from what we would consider a physical or mental illness, Mark gives specific distinctions. Demons often imbue unusual strength to their victims (Mark 5:4) and damage the person they leave (Mark 9:26). The demons also have information the person wouldn't have normally, and verbally protest Jesus' presence (Mark 1:24; 5:7). Those who are healed of physical ailments sincerely seek Jesus' presence (Mark 1:32) and recover gently (Mark 1:31; 5:29).

Mark describes the demon as an "unclean spirit." It is a "spirit" because demons, as well as angels, do not primarily live in the physical world. "Spirit" is from the Greek pneuma, which also means "breath" or "wind," which describes how non-corporeal demons are.

In the ceremonial sense, "unclean" is from the word akathartos: a euphemism for that which is sinful, dirty, and must be avoided if one wants to properly worship God. It represents the opposite state of someone who submits to and serves their Creator. The Mosaic Law lists many things that make someone "unclean" enough to not be allowed in the temple, but not all of them are sin, and it's interesting to note that demon-possessed people in the Bible are not accused of being sinful. Demons, however, are characterized as unclean. They do not worship God, and they should be avoided by anyone who claims to be a Christ-follower.

Jesus does not want His declaration as Messiah to begin through the testimony of an unclean spirit. He silences and banishes the demon to stop it from identifying who Jesus is. Even if a demon has knowledge about God that we don't, it will be sure to use that knowledge to try to thwart God's plan for us. God promises to give us the wisdom we need if we will just ask (James 1:5).