Mat 17:20 Because of your unbelief: for verily I say unto you, If you have faith as a grain of mustard seed, you shall say to this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.

The mustard seed is one of Christ's symbols for the kingdom of heaven, that is, the spirit. It is used because the mustard tree grows so large from such a small seed. "Mounstain" has no symbolic meaning other than its size. This is the first and practically the only time that Christ mentions a mountain in his teaching. The focus of this verse is faith or self-confidence, depending on how you translate it.

"Unbelief" is from apistia, which means "unbelief," "distrust," and a "lack of self-confidence."

"Faith" is from pistis, which means "confidence," "assurance," "trustworthiness," "credit," "a trust," and "that which give confidence."

"Remove" is from metabainô, which means "pass over," "change one's abode," "to pass from one state to another" (i.e. change), and "to make a transition."

"Hence" is from enteuthen, which means "from that place" and "hence."

"Yonder place" is from ekei, which means "there," "in that place," and in philosophy means "the intelligible world."

"Impossible" is from adunateô, which means "to be unable" when it applies to people. It is only applied to things in the NT and then it is translated as "impossible."