Note 4 at Mr 5:9: Mark and Luke recorded Jesus asking the demon his name, but Matthew did not. In 1Jo 4:1-3, we are told to try (test, discern, examine) the spirits to see if they are of God. Discerning of spirits is one of the manifestations of the Spirit mentioned in 1Co 12:10.

According to Mark's and Luke's accounts of this passage, Jesus' command of "come out of the man" was not met with a quick response, as in other cases of deliverance (Mr 1:25-26; Lu 4:33-35, 9:42, and 13:11-13). It appears that finding the "spokesman" for the demons (who identified himself as "Legion") was important in this case. Once the "strong man" (Mt 12:29) of the group was exposed by identifying himself at Jesus' command, the other demons, whose power would have been less than their leader, followed him as he left the man's body.

Whether Jesus actually did not discern the legion of demons or if He did but chose to use this incident as a means to teach His disciples, the demons still knew Him and His authority; the demoniac was delivered. Jesus gave that same authority to His disciples (Mt 10:1, 8; and Lu 10:1-20).