Often a mountain was symbolic in Jewish speech. It represented any insurmountable problem. It was something that they had to live with because there was nothing they could do about it. Jesus told them something different.
The disciples were amazed to see that the fig tree that Jesus had cursed was withered. Fig trees are very hard to kill. What made it more amazing is that one day it had leaves and soon afterwards, it was dead. The tree had withered at the words of Jesus.
Jesus told them that they could say to a mountain that it should be cast into the sea while fully believing it would that this same mountain would be cast into the sea and "splash" there goes the mountain. He tells them to pray and believe so that they might receive.
This can cause us a lot of problems if we think that we can conjure up any desire and have it. Belief has to have a basis. You can't simply believe because you want to believe. You base your belief upon something or someone. Even erroneous beliefs have a basis. If the witch doctor does a dance in the spring and the rains come, you may associate the dance with the coming of the rain. If you want to believe that plane crashes come in three's because everyone says so, you will wait to proclaim they are over as soon as you count three of them.
There are plenty of scripture that says that you won't receive what you want. Was Jesus unbelieving when He asked to have the "cup" removed from Him in the Garden of Gethsemane? Was Paul unbelieving when he asked to have his "thorn in the flesh" removed? No, God is sovereign. Both Jesus and Paul acknowledged this. They knew this before they prayed.
The answer to praying without doubt is in receiving a word from God. God will always do what He has told you that He will do. He speaks to you. You receive His word which tells you what He will do. You believe fully in what He will do. It will happen. Even mountains will be thrown into the sea.
The key is getting a word from God. It is actually more precious than having your mountain thrown into the sea. You know that you will receive what God has said. You can believe and pray without any doubt.
Now, something else very interesting happens. Jesus tells them of their mountains being removed and continues speaking on prayer. He tells them to forgive others when they pray. Now, He doesn't directly say that we won't receive our word from God or that we will not have what God has promised but He infers with the connection of His teaching that forgiveness is directly linked with faith.
It makes sense. You do not understand His forgiveness if you won't forgive others. How can you understand anything the Lord has said if you don't understand forgiveness? In other words, an forgiveness of ourselves and our forgiveness for others is essential for our prayers.
Yes, your mountain can be thrown into the sea. Have you received a word from God telling you that He will do so? Have you believed that word with all of your heart? Have you forgiven all others?
Mark 11:23-25 (NIV)
23 "I tell you the truth, if anyone says to this mountain, 'Go, throw yourself into the sea,' and does not doubt in his heart but believes that what he says will happen, it will be done for him.
24 Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. 25 And when you stand praying, if you hold anything against anyone, forgive him, so that your Father in heaven may forgive you your sins."