“Says to this mountain, 'Be taken up and thrown into the sea'”
Truly I tell you that if anyone says to this mountain, 'Be lifted up and thrown into the sea,' and has no doubt in his heart but believes that it will happen, it will be done for him. (Mark 11:23 BSB)
What it sounds like today
A literal promise of telekinesis that visibly doesn't happen.
The cultural key
Two idioms stacked. The mustard seed is again the proverb for the tiniest possible quantity. And 'uprooter of mountains' (oker harim) was a rabbinic epithet for a teacher who could resolve seemingly impossible interpretive difficulties; 'moving mountains' was already figurative speech for doing the impossible, the way we use it today ('move heaven and earth'). Spoken on the walk between Bethany and the temple, 'this mountain' had a referent in view: from the Mount of Olives one faces the temple mount. Alongside the standing idiom for doing the impossible, many hear a barbed edge aimed at the establishment whose fig tree has just withered.
Recovered meaning
Not a physics claim: 'even the smallest real faith engages God's power with the genuinely impossible.' The contrast isn't small faith versus big faith; it's faith versus none, and a mustard seed's worth suffices.