I think it’s worthwhile noting that some early Gentile Christians had difficulty in interpreting the relationship with the literal and non-literal meanings of certain texts, especially where it interacted with Jewish understandings of God’s omnipotency.
For example, take Mark 10:25 ’ It is easier for a camel to go through the eye of a needle than for someone who is rich to enter the kingdom of God.’ Early Latin Christians translated κάμηλον (camelon, in English ‘camel’) as camellus, but we find in some of their early commentaries an attempt to force an interpretation of camellus as a ‘ship’s rope’ to make the literal meaning more sensical.
Most commentaries (ancient and modern) interpret Matt 17:20 via an understanding that he is using a rhetorical device, adynaton (Gk: ὰδύνατον, in English ‘impossible’) in order further his line of argument. That is, adynaton utilises an exaggerated hyperbole (whether impossible, fantastical or possible but extraordinary) in order to compare and contrast another statement.
In this case, an impossible/fantastical/possible but extraordinary action (moving mountains) is compared to the small size of faith in God (as small as a mustard seed) required to effect that action. The effect of this rhetoric in the context of Matt 17:14-21 is to suggest that the disciples have little faith, and that they need more.
In respect to this pattern of exegesis, it is missing the sense of the passage to focus on ‘moving mountains’ in order to determine whether it is literal or figurative or the degree to which it is impossible, fantastical or possible but extraordinary.
Taking a more prosaic example, if I were to say “I’m so hungry that I could eat a horse” (another example of adynaton), I’m not suggesting that I want to eat a horse, that I could eat an entire horse, that I want horsemeat or that I want as much food as a horse weighs, but only that I’m very hungry. To focus on the ‘horse’ is to misinterpret its function in the context of the line of argument.