Whenever there is a multiple choice of meaning for the translation, context must rule the choice of the English word used. The context of Matt. 21:21 is the meaning that is opposite of "faith", or belief. That would necessarily mean the best fit for "διακριθῆτε" is doubt, that of questioning belief, or unbelief.
And, to get the full meaning of what the Messiah was doing, we need some background from the Old Testament. The tendency of many today is to only consider the literal sense, and they forget that the Bible originated in an Eastern metaphorical mind set.
In speaking of the plagues YHWH brought against Egypt, Psa. 105:33,
"And He smiteth their vine and their fig, And shivereth the trees of their border." (YLT)
In the couplet of Prov. 27:18,
" The keeper of a fig-tree eateth its fruit, And the preserver of his master is honoured." (YLT)
The fig tree is portrayed as bearing fruit and therefore good or beneficial and profitable.
In judgment of the nations, the prophesy of Isa. 34:4,
" And consumed have been all the host of the heavens, And rolled together as a book have been the heavens, And all their hosts do fade, As the fading of a leaf of a vine, And as the fading one of a fig-tree." (YLT)
In the metaphorical language of prophesy, the "heavens" were the rulers and kings of the nations, and their "host" were all of their ruling authorities. Rolling them together as a book... closing the book... was shutting down their kingdoms. The wicked were compared to the fading away, or dying of the fig-tree.
Prophesying of the judgment against Judah and Jerusalem, Jer. 8:13,
" I utterly consume them, an affirmation of Jehovah, There are no grapes in the vine, Yea, there are no figs in the fig-tree, And the leaf hath faded, And the strength they have passeth from them." (YLT)
The picture of a fruitless fig tree was the picture of an unfaithful, and disobedient people or nation.
Now, looking again at Matt. 21, the background is of Christ's entrance and judgment of the temple, casting out all those who had turned His Father's house into a den of thieves (vs. 12-13). That He withered the fig tree was the symbol of the judgment against Jerusalem that was coming upon them, and which He delineated in Matt. chap. 24.
The next symbol is of the mountain they were passing by, most probably that of the Mount of Olives as identified in Matt. 21:1. We need to see how mountains were used in scripture.
In rejoicing at their release from Egypt, in Ex. 15:17,
" Thou dost bring them in, And dost plant them In a mountain of Thine inheritance, A fixed place for Thy dwelling Thou hast made, O Jehovah; A sanctuary, O Lord, Thy hands have established;" (YLT)
Calling to Israel, Isa. 56:7,
"I have brought them unto My holy mountain, And caused them to rejoice in My house of prayer, Their burnt-offerings and their sacrifices [Are] for a pleasing thing on Mine altar, For My house, `A house of prayer,' Is called for all the peoples." (YLT)
God's holy mountain was a sanctuary, a place of worship, or a temple. But, the people turned from His mountain to other mountains of idolatry.
Telling the children of Israel as they are about to conquer the evil nations of Canaan, in Deu. 12:2-3,
"ye do utterly destroy all the places where the nations which ye are dispossessing served their gods, on the high mountains, and on the heights, and under every green tree;
3 and ye have broken down their altars, and shivered their standing pillars, and their shrines ye burn with fire, and graven images of their gods ye cut down, and have destroyed their name out of that place." (YLT)
In 2. Chron. 21:11, speaking of their disobedience and turning away from God to idols,
" also, he hath made high places in the mountains of Judah, and causeth the inhabitants of Jerusalem to commit whoredom, and compelleth Judah."
The mountains were the "high places" of altars to idols, and idolatrous worship; also often called "groves". See High Places
In Ezek chap 20, he remonstrates with Israel for following after idols and idol worship in their "high places". Verses 28-29.
"28 And I bring them in unto the land, That I did lift up My hand to give to them, And they see every high hill, and every thick tree, And they sacrifice there their sacrifices, And give there the provocation of their offering, And make there their sweet fragrance, And they pour out there their libations.
29 And I say unto them: What [is] the high place whither ye are going in? And its name is called `high place' to this day." (YLT)
Contrasting with the mountain of God in verse 40,
"For, in My holy mountain, In the mountain of the height of Israel, An affirmation of the Lord Jehovah, There serve Me do all the house of Israel, All of it, in the land -- there I accept them, And there I do seek your heave-offerings, And with the first-fruit of your gifts, With all your holy things." (YLT)
Again, the reproach of Israel in Jer. 3:6,
" And Jehovah saith unto me, in the days of Josiah the king, `Hast thou seen that which backsliding Israel hath done? She is going on every high mountain, and unto the place of every green tree, and committeth fornication there." (YTL)
Speaking against Babylon, Jer. 51:25,
"Lo, I [am] against thee, O destroying mount, An affirmation of Jehovah, That is destroying all the earth, And I have stretched out My hand against thee, And I have rolled thee from the rocks, And given thee for a burnt mountain." (YLT)
So, the mountains were the places of worship, whether of God, or the peoples' idolatrous "high places". God's mountain where He gave the law to Israel, Mt. Sinai in Exodus 19 - 20, and God's holy city for Mt. Zion in Jerusalem were the exalted mountains of the Lord.
But, the wicked nations, including backsliding Israel were mountains of idol worship, or mountains that would be pulled down. See 1 Kings 19:11; Job 9:5; Isa. 5:25; 34:3; 40:4; Ezek. 38:20; Amos 9:13; Micah 1:4; Nah. 1:5, etc.
Therefore, when Christ told the disciples in Matt. 21:21 that they could move that mountain, the meaning was from the metaphorical usage of mountains as places of idol worship, or of wicked nations who disobeyed God.
The gospel of Christ was the message (Dan. 2:45, Joel 2:32; 3:16, 1 Pet. 2:6) that would break down all "mountains" and all high places, and the disciples of Christ would, through their faith, be spreading that gospel message. Those that responded to the gospel would become the mountain of the Lord (Dan. 2:35, Micah 4:7; Heb. 12:22; ) that would replace all other mountains.
The result that the disciples would see in their lifetime, in the first century AD, was the destruction of the "mountain" of the unfaithful Jerusalem, the same unfruitful fig tree that He withered.
" How beautiful upon the mountains are the feet of him that bringeth good tidings, that publisheth peace; that bringeth good tidings of good, that publisheth salvation; that saith unto Zion, Thy God reigneth!" (KJV)
All bold emphasis is mine.