Mat 14:31 O you of little faith, wherefore did you doubt?

The direct translation of the Greek is:
little trust (oligopistos) for (eis) what (tis) do you doubt? (distazo)

What Christ is asking here is pretty significant. When we believe, we recognize that we believe for a reason, that is, because we get something out of our belief. Christ is saying the the opposite is also true. When we doubt, we must do it for a reason, but what can we get out of it? We do it to protect ourselves. In this case, Peter started to doubt (which means something like "rethinking," see below) because he wanted to save his life, but none of us can save our lives except temporarily. In the end, we are all dead. If we are to gain something beyond death, we have to learn to have faith. Since water is Christ's symbol for the physical, the statement is simply that by trusting in God, we can overcome our physical limitations.

"O you of little faith" is one word in Greek, oligopistos, which occurs only in Matthew. It means literally, "little" (oligo) "trust in others, faith, confidence, assurance" (pistis). It does not have the same sense of religious faith in ancient Greek that it does in modern English. Greek thought balances the risks and rewards of trust. Among their sayings: "Trust (pisties) and disbelief (apistiai) destroy a man." They also said "Trust a thing to destroy. Doubt but preserve."

"Did you doubt" is from distazô , which means "to doubt" and "to hesitate." It means literally, "to drop twice."