This is a passage that is riddled with false assumptions.
Let’s start with the prejudice against people with epilepsy. Straight away Matthew strikes the wrong note. The man's son is not an epileptic. To admit any such thing would be to allow his illness to define him.
Rather, he’s someone who has epilepsy, which sometimes - perhaps occasionally, but perhaps as much as several times a day even - disrupts the normal flow of the son’s life. Nonetheless, he’s not an epileptic. He’s a human being with a disabling condition called epilepsy.
Then again, there is the assumption that epilepsy is associated in some way with darkness and demonic possession. It's certainly a mysterious condition; even today the cause can be unknown, but the prejudice against people with epilepsy is absolutely unjustified. It's a physical condition which can often be controlled very effectively with medication.
The third false assumption is that it's not acceptable to fail. This assumption bedevils Christian mission to this day. Fear of failure prevents many new initiatives or causes them to be abandoned too soon. In the life of the Church, people who’ve failed don't often get a second chance.
In this case Jesus is quite hard on the disciples for failing. He accuses them of having insufficient faith. But it could be argued that at least they had a go.
Someone has argued that where people are in desperate need, being less tolerant of failure can spur us on to help them and so reduce the sum of human suffering. But being more tolerant of failure can also encourage us to try to help people in need, even when the risk of failure is high. Sometimes the risk of failure is worth taking.
So I think we do need to be more, not less, tolerant of failure - especially where people are willing and able to learn from their mistakes or where the risks of failure were already recognised from the very start of the enterprise, but it seemed right to take the risk anyway.
The final false assumption is that great things can be achieved if only we have enough faith. The Gospel passage says that, with great faith, mountains can be moved.
Of course this is sometimes true. The Civil Rights movement in America is an obvious example. Some would say that Brexit is another, but the trouble is that one person's noble cause is another person's idea of craziness or foolhardiness. So even now others are trying with equal fervour to stop Brexit, and are persevering against the odds on exactly the same basis. They have faith in the ideals of the EU and are prepared to go on fighting for them just as the supporters of Brexit campaigned for more than a generation to leave it.
Another problem with the idea that faith can move mountains is that we’ve all seen examples where people did have great faith but the mountains they were trying to move stayed obstinately in their way.
Notice how, according to Matthew's account, Jesus said to the disciples, ‘Truly I tell 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; and nothing will be impossible for you.’ What he didn't say was where the mountains would go. He most certainly didn't promise that the mountains would move to just where people wanted or were expecting them to go, that they actually would move from here to there. By the way he recounts the story, Matthew could be hinting here that it us for God to decide the way in which our prayers will be answered.
Perhaps what we are asked by Matthew to believe is that God will move mountains for usl, because nothing is impossible, but after that we just have to place our trust in God's providence and power. Perhaps that's where the disciples went wrong when they tried to heal the man’s son.
Whatever Matthew may have intended, it's tempting to see Jesus’ talk of moving mountains as mere hyperbole, that is to say a deliberate exaggeration for the sake of emphasis. If true, this would imply that we can't expect to move mountains only to be helped over them.
If we do take the image seriously, as I think Jesus did intend, then - as someone has said - we must be careful not to use it ‘as a pretext for attempting unwise endeavours in the name of the Lord.’ It only makes sense to expect to move a mountain, even a tiny distance, if we are ‘indeed acting in harmony with God’s will.’