What does Mark 5:9 mean?In this verse, most of the attention is usually placed on the name of the demon: Legion. In the Roman army, a legion was a unit of soldiers. At the time of Augustus, this included 6100 foot soldiers and 726 horsemen. The exact quantity of demons is unknown; the name simply indicates a large number working in concert. Typically, less emphasis is placed on the fact that Jesus actually asks the demons for their name. According to ancient pagan beliefs, one's true name represents one's true nature and essence. If that name is used malignantly, it can cause harm. The demons tried to use this supposed power to trap Jesus in a vow in Mark 5:7 when they identified Jesus as "Jesus, Son of the Most High God."
This belief in the power of a name has even led to false teachings about God from those who follow Him. The Jews were so afraid of taking God's name in vain, they relegated it to the tetragrammaton, YHWH, and refused to say it out loud. Today, people in the "Sacred Name" movement believe that using only "Yahweh" for God and "Yahshua" for Jesus shows proper respect. This is problematic, considering that we don't know exactly how "YHWH" is supposed to be pronounced.
Giving someone a name or a new name brings a weight of authority in the Bible. God placed the animals under Adam's dominion (Genesis 1:28) and then told him to name them (Genesis 2:18–20). God changed the names of Abram, Sarai, and Jacob, to Abraham, Sarah, and Israel, respectively. Jesus changed Peter's name, from Simon. All of these reflected their new identities in God's plan. In addition, believers will be given "a white stone, with a new name written on the stone that no one knows except the one who receives it" (Revelation 2:17). Whether that name will be literal or a metaphor for our complete sanctification is unknown.
Some think that Jesus cannot expel the legion of demons until He knows their name. This is incorrect for several reasons. First, there is no indication in other accounts, including Matthew 8:28–34 and Luke 8:26–33, of Jesus using their name, and the demons voluntarily left. In addition, God is omniscient enough to know their name and His omnipotence means He can control them without it. It was more likely a show of force by Jesus who made them reveal what they thought was a hidden vulnerability.