What are unclean spirits?
Question 8037
The phrase “unclean spirit” appears throughout the Synoptic Gospels and the book of Acts, and understanding it requires attention to more than just the Greek vocabulary. The designation is theologically rich, connecting to deeply embedded categories in Israel’s religious life and revealing something specific about the character of the beings it describes.
The Language and Its Setting
The Greek word akatharton means unclean or impure. In the Jewish world of the first century, ritual cleanness and uncleanness were categories that structured every area of life: what could be eaten, who could enter the temple, what contacts were permissible, and how Israel distinguished itself as the holy people of a holy God. Cleanness was not merely about physical hygiene but about fitness for the presence of God. The unclean was that which was incompatible with the divine presence, excluded from the sanctuary, and capable of spreading its impurity to those who came into contact with it.
When the Gospel writers call demons “unclean spirits,” they are deploying this entire matrix of meaning. These beings are not merely hostile; they are fundamentally incompatible with the holiness of God, excluded from His presence by their very character, and capable of defiling those they inhabit. The designation is a theological characterisation as much as a descriptive label.
Usage in the Gospels
Mark’s Gospel uses the phrase “unclean spirit” more frequently than any other New Testament book. The first exorcism recorded in Mark takes place in the synagogue at Capernaum, where “a man with an unclean spirit” cried out at the presence of Jesus: “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” (Mark 1:23-24). The confrontation is deeply ironic: the unclean encounters the holy, and the result is immediate recognition of Christ’s identity and immediate fear of destruction.
The phrase appears in the commissioning of the Twelve, where Jesus gave them “authority over unclean spirits” (Matthew 10:1; Mark 6:7). The authority Christ extends to His disciples is specifically over beings characterised by their impurity.
The Relationship to “Demon”
“Unclean spirit” and “demon” (daimonion) are used interchangeably in the Gospel accounts. In Matthew 12:22-28, Jesus’ act is described as casting out a demon, and the controversy about the source of His authority uses “demon” throughout. In Mark 7:25-26, the text alternates between the two terms to describe the same being in the same account. Luke 8:27-33 uses both “demon” and “unclean spirit” to describe the legion that occupied the Gadarene man. These are not two distinct categories of spiritual being but two ways of describing the same reality, each emphasising a different aspect: “demon” the class of being, “unclean spirit” its moral character.
What the Designation Tells Us
Calling these beings “unclean spirits” communicates that impurity is not merely something they do but something they are. Their rebellion against God has produced a fundamental corruption of their nature, rendering them antithetical to the holiness of God. Where they inhabit a person, they bring defilement. Their presence is incompatible with worship, with the community of God’s people, and with the divine holiness they once served. That the first thing an unclean spirit does in Mark’s Gospel is identify Jesus as “the Holy One of God” is not accidental: the contrast between the holy and the unholy is at the theological heart of the exorcism narratives.
So, now what?
The biblical designation “unclean spirit” is not a quaint first-century idiom but a theologically precise description of the nature of demonic beings. They are defined by their opposition to the holiness of God, and this helps explain why genuine Christian faith and the indwelling of the Holy Spirit are incompatible with demonic possession. The Holy Spirit, by definition, cannot cohabit with the fundamentally unholy. The believer in whom “the Spirit of God dwells” (Romans 8:9) possesses a resident holiness that cannot be shared with the impure. That is not a small comfort; it is a foundational one.
“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.” Mark 1:24