“Pay attention to yourselves! If your brother sins, rebuke him, and if he repents, forgive him, and if he sins against you seven times in the day, and turns to you seven times, saying, ‘I repent,’ you must forgive him.” The apostles said to the Lord, “Increase our faith!” And the Lord said, “If you had faith like a grain of mustard seed, you could say to this mulberry tree, ‘Be uprooted and planted in the sea,’ and it would obey you.”(Luke 17:3-6)
The disciples ask to have their faith increased in response to Jesus telling them how they ought to forgive one another, and Jesus responds with the snark about the mulberry tree in response to their request, which is the disciples making an excuse as to why they don’t forgive as God does.
This is lost in the English translations, most because of how the verses and paragraphs are separated from each other, and the American penchant for proof texting. The result is a faith in faith, rather than a faith in Jesus. People read this and actually think that faith is some mysterious power that can allow you to do superhuman things like quit smoking or some other trivial thing we have allowed to become the definition of piety apart from the word of God. And of course, when they fail they lose faith. Of course, they never had the faith of which Jesus speaks in the first place, because that faith is in the atoning work of Jesus on the cross, not in the ability to supernaturally remove mulberry trees.
No, the disciples hear what Jesus has to say, and then they are traumatized. They don’t forgive others as Jesus has just told them to do. They find it a hard thing to even conceive of doing. Perhaps as impossible as telling a mulberry tree, with all its roots holding it to the ground faster than any other tree holds itself to the ground, to uproot itself and plant itself in the sea. The mulberry tree was famed in Palestine for its ability to hold fast to the ground in which it had taken root.
But Jesus tells them they do not need more faith. They have faith enough to do what he has told them. But refusing to do it, refusing to forgive, they allow sin to remain rooted fast to the stone that is their heart. It is sin that is in the way of them forgiving. It is sin that refuses to forgive. But forgiveness dispels sin, and we have been forgiven by Christ, his death and resurrection. We have what we need to forgive others.