In Mark 11:14, Jesus cursed the fig tree. The next day in Mark 11:20, the curse on the tree was fulfilled. In Mark 11:21, Peter acted surprised about it. In Mark 11:22, Jesus responded to this by saying, "Have faith in God."
Why did the fig tree dry up? Jesus cursed it. If Jesus says the tree is not going to make it, it's not going to make it. A few things here.
One, Jesus is God. His saying, have faith in God, was 'have faith in me' in this instance. Faith in Jesus Christ is faith in God.
Two, have faith in God is have faith in what God says He will do. That's what faith is here, and that is what faith is period. Faith isn't faith in some ephemeral, ethereal, mystical desire you have, treating God like He is a genie-in-a-bottle. Putting faith in God is putting faith in what He says that He will do. It isn't faith in what He can do, but faith in what He will do. How do we know what He will do? He says what He will do, and we believe He will do it, because He said it.
Parallel to 'having faith in God is having faith in what He said' is Romans 10:17, "faith cometh by hearing, and hearing by the word of God." Faith comes from hearing the Word of God. You're believing what you hear Him say in His Word.
Jesus' word here was the fig tree would not survive. When Peter was surprised, this brought Jesus' statement, have faith in God. If Jesus says He's going to do it, then He's going to do it. Faith in God is faith in what He says He will do. It comes by the Word of God. Jesus was expecting Peter and the other disciples at this point to believe what He said. He's God. He can make a fig tree dry up when He says it's going to dry up. A mountain will move when He tells it to move. You should believe it, if He says it, and that's how you believe He will do it.
What is faith in God? It is trusting what God said He would do. It is not trusting what you hope God will do, because you don't know. It is believing Him when He says something He will do. It isn't believing what you want, because you think God can do it. That's not what is happening in this context. You're not having faith if it isn't what He said. That's not faith. Faith is based upon the evidence that is what God says. What God says is evidence, so that you believe that evidence.
Stay with me here, because there is some teaching on prayer here by Jesus in this same text later in Mark 11:25. Jesus said that He will answer the prayer of your believing that you will receive it. Belief is based upon God's Word. What you believe you will receive is what God says you will receive. This is in the context of Jesus saying that the fig tree would be cursed, so that someone should believe that the fig tree would be cursed. Believing is believing what He said would occur.
What can you believe that you will receive? If it is belief, then it is based upon the Word of God. If it is not based upon the Word of God, that is not faith. You are to pray for what you believe that you will receive. God will give it to you because He is not going to go back on His Word. You can believe Him when He says what He will do. Those are the things you ask for. You can't believe that you will receive something that God doesn't say you will. What He promises are the things you should pray for.
There are things that I do not pray for because I do not believe that I will receive them. The things I do believe I will receive are the things that God says I will receive. Those I pray for. The model prayer of Jesus fits this teaching. The prayers of the Bible fit this teaching. Most evangelical and fundamentalist prayers that I hear do not fulfill this teaching.
Having faith in God, we can see, is having faith in what God said. That is what you believe in. That is what you can claim by faith. That is what you pray because you believe you will receive it.