Matthew 17:
15 "Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water.
16 "So I brought him to Your disciples, but they could not cure him."
17 Then Jesus answered and said, "O faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me."
18 And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
19 Then the disciples came to Jesus privately and said, "Why could we not cast it out?"
20 So Jesus said to them, "Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.
Jesus told the disciples to cast out demons, they obeyed, they failed. Faith is, therefore, more than simply obedience to God's word. How is it then that we can do what God says and not have faith? My initial response wants to be that they did not "believe hard enough", that though they had some faith they didn't have enough to get the job done. I believe this fails on two levels.
First, faith is God's gift to us, once given we've got what we need to accomplish the task He's asked us to do. If it was His will that this demon come out, the then He gave the disciples the faith necessary to cast it out. Its God's responsibility to give us faith and He never comes up short.
Secondly, I believe Jesus exhortation to the disciples contradicts this type of thinking. "if you have faith as a mustard seed..." indicates that we need not think about faith in terms of amounts but as a simple having or not having. Either you believe or you don't. Your belief is evidence by your obedience, your unbelief by your shrinking back from obedience.
Thinking about faith in terms of amounts is either emotions or pride. Emotions distort our perception so that the same faith can appear greater or smaller depending on the associating sensations of confidence or fear. Faith is faith regardless of the feelings which come along with it. Pride, obviously, likes the idea of big and small faith. We love to be able to say, "Look at my great big overcoming faith". "I have so much more faith than that person". "My faith is bigger than yours", and so on. Faith is faith, do what God tells you to do and don't worry about whether or not your faith is big or small.
But, back to the main question. Obviously the disciples did not have faith or the demon would have come out. Either they stepped out of God's will and, thus, out of faith as well, or there is more to faith than mere obedience. I don't think that it is the first option or the disciples would not have been rebuked for their inability to cast out the demon. Therefore, there must be more to faith than our obedience alone. I think a lot of it may be due to motives. Trying to walk in God's will for our own reasons will not work. This scripture does not say that this is the case but there are a number of other instances in the Bible in which people try to wield "spiritual power" for their own purposes (Simon the sorcerer) and fail.
I guess I have no real evidence that the disciples were trying to walk out faith for their own purposes but I'm not sure what else would hinder them. I think I'll delve into that another time as it gets into the idea of real faith versus fake faith which is a big ugly subject all its own and I've rambled on for long enough. Let's finish with an expanded idea of what faith is:
Faith is trusting God because He is trustworthy and obeying the things He tells you to do because He tells you to do them, not because of anything you'll get from your obedience in terms of rewards, both material (money, job...) or immaterial (fame, status...).
Obedience is required, but why you obey is important too.