Matthew 21:18-22:

Now in the morning as he [Jesus] returned into the city, he hungered.
And when he saw a fig tree in the way, he came to it, and found nothing thereon, but leaves only, and said unto it, Let no fruit grow on thee henceforward for ever.  And presently the fig tree withered away.
And when the disciples saw it, they marvelled, saying, How soon is the fig tree withered away!
Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.
And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.

Notes on Matthew 21:18-22:

Jesus said that you can move a mountain if you have faith AND doubt not.  Therefore, doubt cannot logically be the opposite of faith.  If it were, Jesus’ statement would make no sense.

If doubt were the opposite of faith, you could doubt and it would mean that you have no faith.  But the Bible says that you DO have faith (Romans 12:3), whether you use it or not, and whether you let doubts drown it out or not.

Thus, it must be possible to have faith AND doubt at the same time.  Doubt in your HEAD would not actually be an issue; only doubt in your heart (which will come out of your mouth) can mess up your attempt to receive by faith.

The tree immediately began to wither, but the time when the disciples saw it was actually the next day (Mark 11:20).

People have read this passage and thought, “I need to really build my faith until it becomes huge mountain-moving faith!”  But the context in Mark’s gospel disproves that.  Right before Jesus talked about the mountain, He said to have faith in God (Mark 11:22).  That’s important.  It is not YOUR spiritual oomph that will move that mountain when you speak to it.  You speak to it because you have faith that you are doing the will of God and that therefore God will back what you say.  You don’t have to produce POWER; you only have to produce WORDS!  The power will follow your words.  You speak to the mountain and God will back your words by moving it into the ocean.  The “pressure to produce” is on Him, not you.   You have the easy part – speaking.  You have faith in God and in His Word that says that you can really do it!  God says that you can move something by speaking to it and He will back His Word!

You might also think to yourself, “That’s a mean thing to do, cursing a poor innocent fig tree.”  But Jesus showed us that we can kill things with our mouths.  That’s very good to know!  At some point you will meet someone with a serious cancer case, and you should be very glad that you have the same right to speak death to that cancer that Jesus had to speak death to that fig tree.

You might also wonder where Jesus got the idea that He could speak death to something and make it die.  I assume that He learned that from Proverbs 18:21.

Notice that Jesus did not “hedge” by muttering under His breath to the tree so that no one else could hear Him in case it didn’t work.  Mark’s account of the story makes it clear that Jesus spoke out loud, as does the obvious fact that Matthew couldn’t record something he couldn’t hear.  You need to speak out loud as well.  If you aren’t willing for other people to hear what you’re saying, you’re not saying it in faith.

Mark 11:14:
And Jesus answered and said unto it, No man eat fruit of thee hereafter for ever.  And his disciples heard it.

See also:

Notes on Mark 11:23-24
Speaking to Mountains, Trees and Diseases
Different Ways to Get Healed
Say What
Objection: Faith Is Simply Trust in God, Not a Force That You Use to Change Things Yourself