Among the hard sayings of Jesus recorded in the Gospels is the "faith the size of a mustard seed" one, which appears in all the Synoptics:
Matthew 17:19-20 Then the disciples approached Jesus in private and said, "Why could we not drive [the demon] out?"
He said to them, "Because of your little faith. Amen, I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you."
Mark 11:21-23 Peter... said to him, "Rabbi, look! The fig tree that you cursed has withered."
Jesus said to them in reply, "Have faith in God. Amen, I say to you, whoever says to this mountain, 'Be lifted up and thrown into the sea,' and does not doubt in his heart but believes that what he says will happen, it shall be done for him."
Luke 17:5-6 And the apostles said to the Lord, "Increase our faith."
The Lord replied, "If you have faith the size of a mustard seed, you would say to (this) mulberry tree, 'Be uprooted and planted in the sea,' and it would obey you."
What makes it hard is that there are a couple billion Christians in the world, and very few mountains being thrown into the sea.
The safest thing is to treat that mountain-moving as metaphorical. The Fathers suggest that the "mountain" in question represents the demons cast out in Jesus' name, or simply any great deed, such as bringing the dead back to life. (More prosaically, we might think of accomplishing great deeds through endurance fed by faith.)
St. John Chrysostom observes of the literal reading:
But if mountains were not removed in the Apostles' time, this was not because they could not, but because they would not, there being no pressing occasion. And the Lord said not that they should do this thing, but that they should have power to do it.
Hard to dispute that, but it still leaves me a little uneasy. I don't believe [stet] a mountain, or even a tree, would move if I told it to. Partly, it's because I can't conceive of a pressing occasion; partly, it's because even if there were a pressing occasion I suspect I'd still feel like I was tempting God or trying to show off; partly it's because I'm not sure faith is supposed to work like that, at least in my life; partly it's because I simply may not have faith the size of a mustard seed.
It's generally understood that faith the size of a mustard seed is not very much faith. In both Matthew and Luke, it's explicitly contrasted with the lesser faith of the Apostles, and we also have the line about the mustard seed being the smallest of all seeds.
But how small is it? St. Jerome, looking at 1 Cor 13:2 -- "if I have all faith so as to move mountains but do not have love, I am nothing" -- concludes, "The faith therefore which is compared to a grain of mustard-seed is a great faith."
I don't think there's a contradiction between some measure of faith being both great and small. Great, relative to the faith possessed by most people; small, relative to the human capacity for faith in the One Who Is both Truth and Love.
Mark's version, included in the peculiar account of the cursed fig tree, adds yet another wrinkle. Not only must the mountain-mover have faith in Jesus, he must believe in his heart that what he says will happen shall be done for him. That is, his faith in Jesus must not only constitute a credal belief that Jesus is Lord and will save him from his sins, raising him on the Last Day to eternal beatitude in the presence of the Almighty. His faith in Jesus must include the belief that, through his faith in Jesus, mountains are his to command.