What does Mark 9:29 mean?
Less-credible manuscripts contain a version of this statement indicating this type of demon could only be expelled with prayer "and fasting." However, those words are not in the older or more reliable manuscripts. Further, Jesus has already established that fasting is inappropriate while He is present (Mark 2:18–20). In Matthew 17:20, Jesus explains the disciples do not have enough faith. Prayer and faith are completely entwined. It is faith that drives us to talk to God, to tell Him our needs, and to express to Him how we rely on Him.Many wonder why we should pray at all if God knows everything we need. That's a selfish view of prayer, as if it is only to compel God to give us what we want. It is often noted that prayer doesn't change God, it changes us. Especially in cases like this, where Jesus' followers are expected to act with the power of God, we need prayer to maintain and strengthen our relationship with God.
This emphasis on prayer shows that faith only works in the context of a relationship with God. We can't make a single decision to follow Him and then forever enjoy the benefits of His empowerment. We don't have the capacity to hold that much strength, like some kind of spiritual capacitor. We must be renewed every day.
Jesus here affirms there are different types of demons with different characteristics, but He does not give details. Passages such as Daniel 10 and Mark 5:10 hint that some demons may be assigned or restricted to a specific geographical location. The mute boy shows that certain demons have specific effects on their victims. But neither apocryphal and pseudepigraphal books on demons nor grimoires and occultic books are to be trusted. Most names given to demons, such as Lucifer, or "Day Star" (Isaiah 14:12–18), and Beelzebub/Beelzebul (Mark 3:22) are metaphors of Satan. "Legion" was so named because it was a group of demons (Mark 5:9). Abaddon or Apollyon is the demonic leader of the demons which are currently imprisoned in the Abyss (Revelation 9:1–3, 11). But reliable information about the specifics of demons is scarce; it's not something Christians should concentrate on.