Q. In Mark 9, the disciples were unable to drive out a spirit and only Jesus could. When they asked why, Jesus replied that this kind of spirit could only be driven out by prayer. Why are there some that are like this, and how can you tell when that is the case?
A. The passage is Mk 9:14-29, with parallel accounts in Mt 17:14-21 and Lk 9:37-43:
• Mk 9:29 And He said to them, “This kind cannot come out by anything but prayer.” Some manuscripts add “and fasting“.
• Mt 17:19-20a Then the disciples came to Jesus privately and said, “Why could we not drive it out?” And He said to them, “Because of the littleness of your faith;
Just as there are different kinds of angels e.g. archangel, cherubim, seraphim, there are different kinds of demons. Some scholars point to:
• Dan 10:13 But the prince of the kingdom of Persia was withstanding me for twenty-one days; then behold, Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia.
• Eph 6:12 For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places.
From the first day Daniel prayed, an angel was sent to give him understanding, but the prince of Persia, a fallen angel, detained him for 21 days. He was not able to overcome the prince of Persia by himself, until Michael the chief prince (archangel) came to his rescue. So there are ranks among demons with different strength, presumably classified as rulers, powers, principalities (Rom 8:38) etc, but the Bible does not give us enough details to identify each.
Although we can not tell which demon is what rank, we do know that Jesus had delegated His authority to His apostles and disciples to cast out demons in His name. The authority is not inherent in them, but His. They can exercise His authority when they have faith, which they manifest by prayerful dependence on Him:
• Mk 3:14-15 And He appointed twelve, so that they would be with Him and that He could send them out to preach, and to have authority to cast out the demons.
• Mk 6:7, 13 And He summoned the twelve and began to send them out in pairs, and gave them authority over the unclean spirits; … And they were casting out many demons and were anointing with oil many sick people and healing them.
• Mk 16:17 These signs will accompany those who have believed: in My name they will cast out demons, they will speak with new tongues;
His authority is available to all true believers. It does not reside in some exorcists as some are led to believe.