It's important to keep the flow of the story in mind. First, Jesus curses the fig tree, which seems odd since it's not the season for figs. This is a signal that this is a dramatic or enacted parable, not an outburst of anger from a hungry man. We understand the point of the parable in the next event, the Temple cleansing.
The Temple cleansing is not simply about religious commercialism - it is a condemnation of the Temple itself. As Wright says, "The Temple has always been an ambiguous thing." Israel knew that it could not be the full and final dwelling place of God. Isaiah and Jeremiah made it clear that Israel would be blessed through the Temple, but that if they used it as a cover for unjust or immoral behavior they and the Temple would fall under judgment. By bringing the sacrificial system to a grinding halt (even for a few minutes) Jesus was acting out God's judgment on the Temple system as a whole.
The next day they passed by the cursed fig tree, which had withered. What does this tell us about the Temple? Just as Jesus had cursed the fig tree and it withered, so would his curse against the Temple bring it to an end. It is at this point that he says:
"Have faith in God. Truly I tell you, if you say to this mountain, 'Be taken up and thrown into the sea', and if you do not doubt in your heart, but believe that what you say will come to pass, it will be done for you. So I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. Whenever you stand praying, forgive, if you have anything against anyone; so that your Father in heaven may also forgive you your trespasses."Judging by the context, the mountain Jesus is referring to is the Temple Mount. This is not a promise that if you pray in faith God will "move your mountains". This is about the passing away of the Temple system and the coming of the kingdom of God in the ministry of Jesus. Read in this way, Jesus' statement reminds me forcefully of the words of the Lord's Prayer: "Thy kingdom come, thy will be done, on earth as it is in heaven." In saying these words we are joining with Jesus in prayer for the coming of God's kingdom and, by implication, the casting down of all competing kingdoms. Since this is prayer according to the will of God we are assured that it will come to pass. But note that this is not to be prayed in a spirit of anger, but with humility and the acknowledgment of our own sins (v. 25). Again, we see the same model in the Lord's Prayer.
In context, then, this exhortation to prayer is not a way to get God to fix our problems (because, if nothing else, experience teaches us that God does not always do that), but a participation in the work of Jesus. As Jesus predicted, the Temple system passed away. Yet, other systems oppose themselves to the kingdom of God in our day. We have this promise that they, too, will fall.