A somewhat lengthier discussion is this challenge on the site "God is imaginary":*
In Matthew 17:20 Jesus says:
For truly, I say to you, if you have faith as a grain of mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible to you.
If "nothing will be impossible to you", then if we ask to cure cancer tonight, cancer should disappear. Right? Yet nothing happens. Note that if we take the Bible less-than-literally here, the statement "nothing will be impossible to you" becomes "lots of things will be impossible to you," and that would mean that Jesus is lying.
Now, what does Haydock comment have to say?
ST. MATTHEW - Chapter 17:19** Jesus said to them: Because of your unbelief. *For amen I say to you, if you have faith as a grain of mustard-seed, you shall say to this mountain: Remove from hence to yonder place, and it shall remove, and nothing shall be impossible to you.
Ver. 19. If you have faith as a grain of mustard-seed. Christ insinuates to his apostles, as if they had not yet faith enough to work great miracles, which require a firm faith joined with a lively confidence in God. The mustard-seed is brought in with an allusion to its hot and active qualities. (Witham)
That is, a perfect faith; which, in its properties and its fruits, resembles the grain of mustard-seed in the parable. (Chap. xii.[xiii.?] 31.) (Challoner)
By faith is here understood, not that virtue by which we assent to all things that are to be believed of Christ, the first, of the theological virtues, in which the apostles were not deficient, but that confidence in the power and goodness of God, that he will on such an occasion exert these, his attributes, in favour of the supplicant. To have a true faith of this kind, and free from all presumption, is a great and high privilege, which the Holy Ghost breathes into such only as he pleases. (Jansenius)
Examples of this efficacious faith are given by St. Paul. (Hebrews chap. ii.[xi.?]) St. Gregory of Neo-Cæsarea is also related, by Eusebius and Ven. Bede, to have removed by the efficacy of his faith a rock, which obstructed the building of a church; thus literally fulfilling the promise of Jesus Christ. (Tirinus)
The faith of the apostles, especially of those that had not been present at the transfiguration, was not perfect and complete in all its parts, till after the resurrection and ascension of Jesus Christ, and the descent of the Holy Ghost. (Haydock)
St. Jerome understands by mountains, things the most difficult to be effected.
One simple example of the fact that the Apostles eventually had the faith to move mountains: 100 years after Spartacus and his followers were beaten in battle and crucified along the Via Appia or whereever it was, it was as impossible as anything to imagine slavery would disappear.
Yet slavery did disappear where Romans and Franks ruled one after another. And in the rest of the Occident.
Now for the whole context:**
14 *And when he was come to the multitude, there came to him a man falling down on his knees before him, saying: Lord, have pity on my son, for he is a lunatic, and suffereth much: for he falleth often into the fire, and often into the water.
15 And I brought him to thy disciples, and they could not cure him.
16 Then Jesus answered, and said: O unbelieving and perverse generation, how long shall I be with you? how long shall I suffer you? Bring him hither to me.
17 And Jesus rebuked him, and the devil went out of him, and the child was cured from that hour.
18 Then came the disciples to Jesus secretly, and said: Why could not we cast him out?
19 Jesus said to them: Because of your unbelief. *For amen I say to you, if you have faith as a grain of mustard-seed, you shall say to this mountain: Remove from hence to yonder place, and it shall remove, and nothing shall be impossible to you.
20 But this kind is not cast out but by prayer and fasting.
In other words, everywhere an exorcist succeeds, there is an example of this promise in work. Ask Gabriele Amorth of his experience thereof.
This is no warrant for just any layman - the Apostles to whom Christ spoke were the first bishops of the Church - asking anything and waiting for it to happen and it will.
But as Tirinus commented, one bishop had in fact done what Christ promised. St. Gregory of Neo-Cæsarea is also related, by Eusebius and Ven. Bede, to have removed by the efficacy of his faith a rock, which obstructed the building of a church; thus literally fulfilling the promise of Jesus Christ.
Bpi, Georges Pompidou
or First Sunday of Lent
* God is imaginary : Proof #1 - Try praying
** Haydock's Catholic Bible Commentary, 1859 edition. ST. MATTHEW - Chapter 17