“snatched up.” The Greek is airō (#142 αἴρω; pronounced eye-rō), and it is passive voice, imperative mood. Although it would be very literal to say, “Be taken up,” the imperative mood combined with the context, moving a mountain at your command, gives the sense that the mountain is being snatched up out of its place and thrown into the ocean (cp. The Source New Testament, which also uses “snatched”).
“sea.” In this context, Jesus is teaching in Jerusalem, and the Mediterranean Sea and Dead Sea were the closest and best known bodies of water.
“does not doubt.” The Greek is diakrinō (#1252 διακρίνω). In the middle voice, as it is here, it refers to being undecided within oneself. It is the indecision that causes one to hesitate or waver. Nyland (The Source New Testament) makes the case that “doubt” is not a good translation here, saying apisteō or aporeō would be “doubt,” and “undecided” would be better. While it is true that we often use “doubt” in the sense of a specific and steady state of mind, such as when we “doubt” that what someone says is true only because we cannot “prove” them to be lying,” it is also true that we use “doubt” of the times we doubt ourselves and waver between doubt and trust. Also, “undecided” might seem to say we are undecided about obeying God, which is not what the verse is saying.
This verse makes a strong point about the manifestation of trust (which is the full context here. It takes revelation from God, and then the manifestation of trust to move a mountain). When God gives us revelation that something can be done at our command, of course it will not happen if we doubt we can do it. But even if we are “undecided” and waver between trust (faith) and unbelief, we will not be able to carry out the will of God. Like Abraham, we must be strong in our faith, our trust in God. [For more on the manifestations of holy spirit, and the manifestation of trust, see commentary on 1 Cor. 12:9].