A legion was a large unit of the Roman army. In New Testament times, the regular size of the legion was 6,000 men. In addition to these men, some 120 cavalry were added.
Because it was a large body of men, the word “legion” came to represent a generally large number. It is used this way four times in the New Testament. In the story about the demon-possessed man in the country of the Gerasenes, Jesus asked the man, “What is your name?” The man replied, “My name is Legion, for we are many” (Mark 5:9, 15; Luke 8:30) Matthew 12:45 and Luke 8:2 speak of a number of demons possessing a single individual.
Another use of the word is found in Matthew 26:53. At the time of Jesus’s arrest, one of those with him drew his sword to defend Jesus. Jesus spoke against such action saying, “Are you not aware that I can call on My Father, and He will at once put at My disposal more than twelve legions of angels?” He spoke of the large number of angels that could be called to help him.
The word “legion” is never used in the New Testament in its military sense. It is used for either the spiritual powers of evil that oppose men (Ephesians 6:12) or the spiritual powers that can be called to their aid (Hebrews 1:14).