What can God do, and what will He do? Those are two profoundly different questions. We know, in theory, that "all things are possible with God." All Christians agree with that. But beliefs vary tremendously when we talk about what things are probable with God.
All things are possible with God, but the things God will do that we consider "probable" are what we actually have faith to see happen. And through our faith, we have a profound influence over the things we will see God do.
As Jesus was leaving the area, two blind men followed Him, shouting, "Have pity on us, son of David!" at the tops of their voices. Jesus went into the blind man's house, and the blind men came to him. "Do you believe that I can do this?" Jesus asked. "Yes, Master," they replied. Then Jesus touched their eyes. "As you have believed, so let it happen," He said. And their eyes were opened.
Matthew 9:27-29 (emphasis added)
Before Jesus performed this miracle, He asked the blind men if they believed He could do this. It wasn't enough for these men to give a general assent to the notion that God could, in theory, do whatever He likes. Jesus specifically wanted to know if they believed He could provide them with sight. Jesus then healed them according to what they believed He could do.
Faith is not an emotion that we work up to get God to do things. Faith is actual, feet-on-the-ground confidence that God can and will do something. I know, in theory, that God could make me fly if I asked Him. But I have no faith that God will make me fly at this moment. There is an ocean of difference. To see the power of the Gospel in our world today, we must begin to cross the ocean of theory and arrive in the land of practical application. We must start developing our faith through actions.
There is a real sense in which we determine what God will do in our lives and our world through our faith. The way Jesus worked miracles provides evidence of this truth:
...suddenly an official came up and knelt down in front of Him. "It's my daughter!" he said. "She's just died! But — if you'll come and lay your hand on her, she'll come back to life!" Matthew 9:18
A short time earlier in Matthew's Gospel (Matthew 8:5-13), a centurion came up to Jesus with a similar request for his dying servant. But in that case, the centurion told Jesus, "I don't deserve to have You come under my roof! Just say the word, and my servant will be healed." Matthew 8:8 Jesus' response is to say to the centurion, "Go home. Let it be done for you as you believed." (Matthew 8:13)
In the second case, Jesus could have told the officer that He didn't need to come to touch his daughter for her healing. Jesus could have told the official that He could perform remote healings as in the case of the centurion's servant. But instead, Jesus did precisely what the officer asked Him to do.
"Let it be done for you as you have believed."