Could you tell me what Matthew 21:21 means: "if you have faith and do not doubt, not only can you do what was done to the fig tree, but also you can say to this mountain, 'Go, throw yourself into the sea', and it will be done"?
With the words of this passage, therefore, our Lord cuts right to the quick on the issue of what it means to truly believe. The same sentiment is found at Luke 17:6, Matthew 17:20, and 21:20-21, and although the passage in Luke speaks of a fig tree being uprooted and planted in the sea, there and in the Matthew 17 passage Jesus tells us that we will receive these dramatic answers to prayer if only we have the tiniest amount of faith (as small as a grain of mustard seed). Further, in the Matthew 17 passage our Lord is explaining why the disciples were unable to cast out the demon which He did cast out after His descent from the mount of transfiguration – "because of your little faith".
"Not by might nor by power, but by My Spirit" says the Lord of Hosts. What are you, O great mountain? Before Zerubbabel (i.e., a type of Christ) you will turn into a plain (i.e., for the construction of the temple). And He will bring forth the Corner Stone to the shouts of "grace, grace be upon it!"
As this passage in Zechariah shows, moving mountains – whether literal or metaphorical – can only be accomplished by the Spirit of God in the will of God (cf. Job 9:5 – God is the only one who can do this, clearly). In Matthew 21:22, in the context of the verse you ask about, Jesus adds, "If you believe, you will receive whatever you ask for in prayer". Therefore the challenge of uprooting the fig tree and moving the mountain represents where we as believers in Him who promises us this should be - that is, at the point in our spiritual lives where we truly do possess supreme confidence of faith, a state that is only achieved through a) a mature knowledge of what God's will is (Phil.1:9-10), and b) a mature relationship with Him (achieved only through spiritual growth based on said knowledge; cf. Heb.5:14). Our Lord had both of these to a perfect degree, developed over a life-time of perfectly trusting God and resolutely and consistently growing in grace through diligent spiritual growth (Lk.2:40; 2:52). For this reason, His ability to "move mountains" through prayers of faith cannot be compared to the "little faith" often demonstrated by the disciples (cf. Matt.17:19ff), and all too often by us as well. But the closer we get to Him, through faith, the more our faith strengthens and the more effective our prayers become. If we cannot move mountains now, everything our Lord says to us indicates that we can develop our faith to such a degree, and that even "a mustard seed" worth of faith is enough to do the most amazing things.
Many people think that this faith necessary for "mountain moving" is a matter of working something up - like rallying one's courage in a difficult situation. But in truth it is more of a letting go than a "summoning up". Inevitably there will be many times and circumstances in all of our lives when there is nothing we can really do about a particular problem, no matter how hard we try (or plan or think). At such times, we need to learn to trust God that He will resolve everything according to His will, remembering and believing that that will is for our good in every way (Rom.8:28). When Abraham took Isaac up to the mountain to sacrifice him, he had no idea how this situation could possibly work out for good - after all, it was God Himself who had ordered the sacrifice so that it was not even a case of waiting for God to weigh in on the problem, at least as far as Abraham could tell - this is a critical point that is not often recognized, and explains the depth of Abraham's obedience and trust in the Lord (so that he was truly God's "friend"). When Daniel was taken to be thrown into the lions' den, he had no idea how God or even if God would deliver him, but we know that he put the matter into the Lord's hands in the face of that terrifying and awful prospect. After all, he was about to be horribly killed - absent some unforeseen miracle - and precisely because he was persistent in faithfulness. Yet scripture tells us that he was delivered "because he had trusted in his God" (Dan.6:23). In both of these cases (and countless others in scripture and in our own experience and observation) there was simply nothing the believer could do but continue to believe - pray, to be sure, but pray in complete faith of God's help, God's deliverance, and the accomplishment of all God's good will, no matter what the situation may have looked like from our limited, earthly perspective.
Moving a mountain is impossible - for anyone but God, but nothing is impossible for God. He is the true focus of these verses, not the miraculous event contemplated, or even the person who prays for it with such great faith. Should this actually come to pass, it would not be the miracle itself that should attract our attention, nor should the person who makes this petition be held in excessive awe, but rather God who brings such glorious things about for His own sovereign purposes. Moving a mountain is impossible, and that is the point of Jesus' comparison. We need to understand that if we need a mountain to be moved, then God is not only capable of doing this with no effort, but He will do whatever is necessary and good and right in the fulfillment of His will and plan for our lives. He can and will do whatever is needful - we only have to believe it. We all have mountains, obstacles in our paths that are so large and so daunting that surmounting them is an obvious impossibility. But if we remember our Lord's words here, we know that He has us by the hand, that we won't sink in the rising waters as long as we continue to trust Him.
Paul makes reference to this same sentiment in 1st Corinthians 13:2 where he speaks of "faith to move mountains" - clearly this represents a supreme test of faith. Is my faith strong enough to believe that God will do the impossible for me? We are not being asked to believe that God will do absolutely anything for us no matter what it might be - we are not nearly smart enough to know what is best for us even when we are asking with entirely pure motives - rather we are being told that God will do absolutely everything for us that we need and that we ask for in faith, even if we don't know really know what to ask for (Rom.8:26). For it is not the knowledge of our circumstances that is being commended here by implication but faith even in (or perhaps especially in) the absence of knowledge about what our circumstances truly require. If we doubt, we should not expect God to answer these "mountain moving prayers" (Mk.11:23; cf. Jas.1:6-8), because when we doubt, it is really God's character, God's promises, and God's ability that we are doubting. In all such testing, whenever we find ourselves staring up at the snow-covered Rockies without any means of passing, we need to remember that God knew we would be standing here, and He knew we would need a way through - that is at least part of the reason that the mountains are there in the first place (and probably the biggest part). Without mountains in the way, our faith would never be put to the test to a degree necessary to make it grow on the one hand, and to test its true worth on the other. Mountains in our path are God's way of refining our faith like gold, and also of assaying its true value for all the world to see, men and angels both (Jas.1:2-3; cf. Rom.5:3-5; Heb.11).
It is true that our Lord puts the matter in these verses you ask about in a very matter of fact way. Like so many of His words, this has real "steam". We put ourselves in the situation and blanch. He tells us, "just believe, and it will happen". We know that there is pain involved in these things, and time too, often great trackless vistas of time before the mountains move, but our Lord is forcing us to look to the end of the matter. Even if that mountain moves glacially slow, at the end, if we have trusted Him, we have our victory, and we see that He did move it. From our human perspective, mountains usually move imperceptibly slowly when they move at all, but with God one day is as a thousand years, and the whole span of human history is but the blink of an eye. We know these things, and we know that we can trust Him to do even the impossible things we need Him to do. But we need more than knowledge - we have to put what we know about Him, His goodness, His power, His foreknowledge, His mercy, His planning out of all things, to work in our hearts with complete child-like faith, that ignores what is seen and felt, and trusts rather in the words of God.
After all, we already have the experience of this. To save us from death and destruction, we needed someone to die for our sins - the impossibility of impossibilities - and God provided this at the utmost cost to Himself. We have the cross of Jesus Christ to remind us that, compared to the sacrifice on Calvary, every other mountain is infinitesimally small.
And one last important observation on this. Jesus' comment on moving mountains occurs in the context of His disciples being unable to cast out a particularly stubborn demon (Matt.17:18-20). That feat our Lord accomplished instantly - and it was instantly needed and necessary. Our trouble often hand heavy upon us and we want them solved instantly in like fashion, but "waiting on the Lord" is also something we need to perfect. There are some mountains which must be moved immediately for us to be delivered and/or accomplish the objective our Lord has for us. In many cases, however, moving that mountain may require waiting on the Lord to answer our prayers to that effect at just the right time. In such situations, the measure of our faith is taken not only in the degree to which we have confidence in His ability to do it, but also and importantly in our persistence in that belief even when it does not happen as quickly as we would like.
For more on faith, please see these links:
Faith Dynamics (Peter #24)
Free-Will Faith and the Will of God.
Faith: What is it?
Free-Will Faith in the Plan of God.
Please see also in CT 5 "Jesus Christ's Return to Earth on the Mount of Olives (Zechariah 14:2-7)" where this promise is literally fulfilled.
You can find out more about prayer at the following links:
Corporate Prayer: "When Two Agree on Earth" (Matt.18:19)
The Lord's Prayer
Can Prayer be Offered to the Son?
Holding up Holy Hands in Prayer
Praying for Wisdom
Prayer for Failing Faith
Can Prayer Be Offered From Heaven?
Prayer and Imitating Christ
In Him who is able to do abundantly beyond whatever we ask or think, our precious Lord and Savior Jesus Christ.