Mat 21:21 Truly I tell you, If you have faith, and doubt not, you shall not only do this [which is done] to the fig tree, but also if you shall say to this mountain, May you be removed, and be cast into the sea; it shall be done.

The most interesting term here is "doubt," which is from a word that means "to divide." The translation "to doubt" is attributed only to its use in the New Testament. There are a number of Greek words that do mean "doubt" (for example apisitia, apisteô, literally, "without faith") but the term Matthew uses means "divide." In cases such as this, my tendency is to think that Christ really said "divide" and not "doubt."

Christ here is referring to our human tendency to second-guess ourselves and to divide our focus. In general, most people lack the ability to focus on a goal without questioning themselves. Instead, we hedge our bets, make alternative plans and readily accept alternatives. This puts our goals in doubt.

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

"Doubt" is from diakrinô, which means "to separate one from another," "to distringuish," "to decide," and "to separate into elemental parts."

"Be removed" is from aeirô (aero), which means "to lift up," "to raise," "to raise up," "to exalt," "to lift and take away," and "to remove."