Adam Clarke on Matthew 21:21

If ye have faith, and doubt not - See on Mat 17:20 (note). Removing mountains, and rooting up of mountains, are phrases very generally used to signify the removing or conquering great difficulties - getting through perplexities. So, many of the rabbins are termed rooters up of mountains, because they were dexterous in removing difficulties, solving cases of conscience, etc. In this sense our Lord's words are to be understood. He that has faith will get through every difficulty and perplexity; mountains shall become molehills or plains before him. The saying is neither to be taken in its literal sense, nor is it hyperbolical: it is a proverbial form of speech, which no Jew could misunderstand, and with which no Christian ought to be puzzled.