Really? I memorized that verse, Matthew 21.22, some years ago, but I don't really understand it. I mean, the Bible says that asking with wrong motives will mess things up (James 4), and that asking according to God's will (1 John 5) is important. But here in Matthew, it just says, “If you believe, you will receive whatever you ask for in prayer.” (Matthew 21.22). How can this be?
Here's my take on it. How can we have this kind of faith? How can we be so sure (verse 21 has if you have faith and do not doubt) that what we're praying will happen? I have two possible answers.
First: this comes from God; he might speak to someone in a way that is utterly convincing: "I will do this."
Second: this verse shouldn't be taken alone to form your understanding of prayer. Other verses, from James and John among others, and the experiences of the saints recorded in Acts and in the letters (I'm thinking especially of 2 Corinthians 12) must also be considered. This isn't exactly a cop-out; Jesus is recorded as saying that "Anyone who divorces his wife and marries another woman commits adultery against her" (Mark 10.11), but Matthew 19.9 has him offering an exception: "I tell you that anyone who divorces his wife, except for marital unfaithfulness, and marries another woman commits adultery." (Matthew 19.9).
So it appears that Jesus sometimes says things that he intends for us to take along with other things. He sometimes seems to say extreme things to make a point, and the trouble is that we think he's making a dozen points when he's only making one or two.
Looking at the context, it's right after the fig tree incident and before the elders confront him on his authority. That makes me think his point is this:
The most powerful force that mattered to Jesus's disciples was the existing religious order. This order was opposed to Jesus and his followers, and appeared to be immovable - like a mountain. I think he was saying that no force in all the universe was too powerful to thwart the will of God and the power of prayer. And that faith with prayer would be required -- probably you need help from God in that department, too (I certainly do).
And fortunately, our God is willing and able!