What if it is a secret? So the question becomes how can we know for sure whether or not the Mark 9:29 statement belongs in the Matthew passage? The Mark passage does not mention the mountain in the immediate context as does the Matthew passage.
Matthew 17:19-21 ASV
19 Then came the disciples to Jesus apart, and said, Why could not we cast it out?
20 And he saith unto them, Because of your little faith: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
21 - - -
Mark 9:29 ASV
25 And when Jesus saw that a multitude came running together, he rebuked the unclean spirit, saying unto him, Thou dumb and deaf spirit, I command thee, come out of him, and enter no more into him.
26 And having cried out, and torn him much, he came out: and the boy became as one dead; insomuch that the more part said, He is dead.
27 But Jesus took him by the hand, and raised him up; and he arose.
28 And when he was come into the house, his disciples asked him privately, How is it that we could not cast it out?
29 And he said unto them, This kind can come out by nothing, save by prayer.
Therefore we must mesh these two passages by reading them together in order to obtain the greater message. Is the Matthew passage teaching that one with enough faith can move literal physical mountains? Surely not, for who has ever accomplished such a thing? However we now have much more background context than just these two passages. In fact the Gospel of Matthew comes to this topic once again, four chapters later, when the fig tree is withered.
Matthew 21:18-22 ASV
18 Now in the morning as he returned to the city, he hungered.
19 And seeing a fig tree by the way side, he came to it, and found nothing thereon, but leaves only; and he saith unto it, Let there be no fruit from thee henceforward for ever. And immediately the fig tree withered away.
20 And when the disciples saw it, they marvelled, saying, How did the fig tree immediately wither away?
21 And Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do what is done to the fig tree,
but even if ye shall say unto this mountain,
Be thou taken up and cast into the sea, it shall be done.
22 And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.
Ahah! we've struck gold, we've found a pearl of great price hidden in a field: for herein he mentions the mountain yet again and says that essentially the fig tree analogy is of the like nature, (in understanding), and herein the mountain is cast into the sea! That's prophetic language for sure. These things are being spoken in parables, proverbs, idioms, and types, (typology). Thus, indeed, the Mark 9:29 statement belongs in the Matthew 17 passage, (at Matthew 17:21). What therefore are these
evil mountains? Just as nearly all things Biblical there is good and there is evil when it comes to typology, so just as there are the good and holy mountains of Elohim, (Horeb, Zion, etc., etc.), there are also seven
evil mountains greater and mightier than Yisrael.
1) Revelation 19:10 (the Spirit of the prophecy is the Testimony of the Meshiah)
2) Revelation 8:8 = Matthew 21:21 =
Henok 18:12/13
3) Revelation 17:9-10 = Genesis 8:5 =
Henok 18:12/13
4) Genesis 8:5 ~ rashei = "tops" or "heads"???
This is an immersion, 1 Peter 3:20-21, and thus the evil mountains have heads, for the seven heads are seven mountains, and there are seven kings, (and the seventh is Legion), and the eighth is of the seven, (because he is the first, just as Tubal-Qain brings forth Qain).
Therefore Matthew 17:21 belongs in the text, whether it was there originally or not, (for it may have been a secret only meant to be expounded in another account, such as in Mark), for he speaks of supernal and spiritual things. Moreover these things speak of a great immersion, in the typology of the flood, and the sea is for the Kohanim to wash in, and thus, once the immersion is complete and the talmid is fully cleansed, there is no more need for the sea, (Revelation 21:1).
PS: of course, this also has to do with your current stars thread.