I believe this passage by Elizabeth Elliot uncovers why so many Christians are stagnant in their walk. Their faith isn't growing because they simply won't obey the Lord. If you want a bigger faith, obey God's commands. No, it isn't easy. In weight training, one learns one has to tear the fibers in a muscle before strength is gained. Faith is very similar. You have to surrender your will, obey, and then your faith will grow.
There is something that pretends to be Christianity which is mostly a mood. The measure of its faith is merely the measure of feeling. ...
That faith is neither a mood nor a feeling, but practical obedience is clearly seen when Jesus puts the responsibility of forgiveness squarely before the apostles--seven times in one day.
He told them they must forgive the same man. They recognized dimly that this was going to require faith and theirs did not amount to much. Mood and feeling would not go far toward enabling them to obey that command. "Lord increase our faith!" they said, very likely taking refuge from obedience by implying that they could not possibly be expected to obey until, by some special miracle of grace, they were given a superabundance of faith. Superabundance indeed! Why, even faith no bigger than a mustard seed could uproot a mulberry tree, Jesus told them.
The way to increase their faith was to get busy and do what they were supposed to do, and "when you have carried out all your orders, you should say, 'We are servants and deserve no credit; we have only done our duty." Elisabeth Elliot
