If you want to know the true meaning of the Parable of the Mustard Seed, you must take it in the context of the Parabolic Teaching itself, and then compare it with the rest of the symbolism it touches upon in the Word of God.
The first principle of interpreting the Parables is given in the first one he spoke in Mark where, if they did not understand the Parable of the Sower, they would not understand any of the Parables.
The second principle that one must remember is that Y'shua did not start speaking in Parables until it was obvious that he had been rejected by the Leaders of the Nation (per the commentaries) and, therefore, the parables are refering to the Postponement of the Kingdom and then its subsequent restoration at the end of the Age.
A Third principle is simply that neither Y'shua, or the Holy Spirit, are needlessly repetative and, therefore, the parable of the Mustard Seed is not simply a rehash of the Parable of the Sower.
So, with these principles in mind we can compare the Word.
The Sower is Y'shua (i.e. "the Son of man") and the field is "the World" however, some accounts say it was sown in "the garden" which, then, can only be Israel (as God's Vineyard etc per the prophets).
The Seed is "taken" which is the same word as the Rapture and is something to keep in mind.
A specific person who has the faith of a Mustard Seed will say to a Mountain (symbolic of a ruling Kingdom) and a Sycamore Tree (an inferior Fig Tree and thus can only be the endtimes Den of Vipers - which itself originated in Babylon via the Kabbalah etc., returned for judgment day and is synonomous with the "fig tree" of the other parable that the son of man visited "these three years" or the first three years of the tribulation), "Be thou removed and cast into the sea" and it will be done.
When we search the Scriptures we see that this is exactly what is prophecised of Zerrubbabel in Zechariah who symbolizes someone in the future during the tribulation.
Further, being the Least of all seeds, brings this concept into play as well and if you do a search of "least" in the Scriptures, you will see this as well.
"The Least of the Flock will draw them out" refering to the destruction of Babylon in the end days, which is echoed by the "first born of the poor" (i.e. the poorest of the poor of, thus, the least of the least) who will also destroy "the rest" of the Chaldeans.
Thus, here we see, when we compare these (and other Scriptures), that the Mustard Seed, through faith in Y'shua ben Y'hova will be responcible for overturning the endtimes Mountain of Babylon at the return of the Captives of Israel, when they will receive "the headstone thereof" as their Messiah and Elohim - Y'shua.
Thus, what you obviously have with this parable is the restoration of the Theocracy in the end days which is one of the reason the Mustard Seed was used to illustrate all this for it is a fast growing herb and sprouts leaves in 48 hours to show the speed of the restoration of the Theocracy during the tribulation.
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