Matthew 17:21, This kind [of demon]. There are different kinds of demons that influence humans. The Testimony of Yeshua speaks of being “possessed with a demon” or “demons,” “vexed with a demon,” or “severely possessed with a demon” (Matt 4:24; 8:16, 28, 33; 9:32; 12:22; 15:22; Mark 1:32; 5:15, 16, 18; Luke 8:36; John 10:21).
All these ideas are expressed by the Greek word daimonizomai meaning “to be under the power of or possessed by a demon, or demonized.”
There are levels of demonization. The Bible reveals that demons can influence humans (e.g. from fiery darts of Satan shot at the saints by which he attempts to influence them [Eph 6:16], by lying to humans [1 Kgs 22:22–23], through unclean spirits [e.g. Matt 12:43; Mark 1:23, 26; 3:30] and by literally living in or possessing a person. The latter is the most extreme form of demonization.
In this passage, a demon lived inside of a child (v. 18), and when Yeshua rebuked the demon, it came out of the child. When a person is possessed with a demon, prayer and fasting may be necessary on the part of the deliverer to cast out the demon. This is because the deliverer has, to one degree or another, himself been weakened spiritually by faithlessness and perverseness and therefore lacks the spiritual power and faith to cast out the demon (vv. 17, 22), or because he has been negatively influenced by those around him who are this way (v. 17), and thus has been weakened by his spiritual environment. This is why Yeshua, at times, had to put the spiritually weak and faithless people out of the room before healing a sick person (Mark 5:40).
Prayer and fasting energizes a person spiritually by afflicting the soul (the mind, will and emotions) and forcing it into conformity with the word, will and Spirit of Elohim through which one gains the spiritual power to accomplish supernatural feats such as casting out demons.
When Yeshua speaks of “this kind” of demon (v. 18). What did he mean? What kind of demon? The Bible reveals that there are various levels and types of evil, satanic spirits (e.g. Eph 6:12). The Book of 1 Enoch teaches and second temple Jewish sages based on this that demons were the disembodied spirits of the nephilim (see Gen 6:2–4), who after their human bodies died continued to live (as ghost-like figures) roaming in search of a body (human or animal) or object to possess (1 Enoch 15:8–10). This may be what Yeshua is referring to here.
With regard to the origin of demons, the 1906 Jewish Encyclopedia under the articles entitles “Demonology” states,
The speculation regarding the nature and origin of these demons and their leaders led as early as the second pre-Christian century, in those fragments preserved under the name of the Book of Enoch, to the story of the fall of the angels (Enoch, vii.-viii.; lxix.). Like Beelzebub, or Lucifer (Isa. xiv. 12; compare Slavonic Enoch, xxix. 4), two hundred ‘Irin or “watchers” fell, attracted by the beauty of the daughters of men (Gen. vi. 4); only tradition obviously differed as to the leader of the rebellious host, whether it was Azazel or Shamazai. At any rate, they acknowledged the supremacy of Satan (liii. 3, liv. 6), though occasionally many satans are mentioned (xl. 7 et al.), and these fallen angels became “the evil spirits” (xv. 8, xix. 19) who taught mankind all the arts of deception, witchcraft, and sin (vii.-viii., lxix.). But their children, the offspring of this mixture of an earthly and a celestial race, became, when slain, the hybrid race of disembodiedspirits or demons doing the work of destruction until the Day of Judgment (xvi. 1).