In a number of places in the gospels, Jesus describes his disciples as having ‘little faith’. For example, after a failed attempt by the disciples to set a boy free from his oppression, Jesus explains to them that were unable to do so because they had ‘so little faith’ (Matthew 17:20). Similarly, when Peter attempts to walk on water, and begins to sink, Jesus is forced to reach out his hand saying, ‘You of little faith’, ‘why did you doubt?’ (Matthew 14:31). In fact, the term ‘little faith’ is one word in the Greek, ὀλιγοπιστίαν (oligopistian) in 17:20 and ὀλιγόπιστε (oligopiste) in 14:31. Furthermore, whilst most Greek words in the New Testament can be found in other ancient Greek texts, this word cannot – leading some to believe that Jesus may have made it up!
Whilst the word ‘oligopistian’, or one of its forms, is not found outside of the New Testament, its constituent words are - πίστις (pistis) which simply means ‘faith’, however, there is some argument as to what ‘oligo’ means in this context. (the KJV renders ‘little faith’, in 17:20, as ‘unbelief’, but elsewhere uses 'oligopistian' or a form of it - unlike modern translations the KJV New Testament relies heavily on the Greek Textus Receptus of Erasmus - it seems to me, more understandable that Jesus was not telling off his disciples for having ἀπιστίαν, apistian 'no faith', but rather ‘oligo’ faith). ‘Oligo’ itself can be translated as ‘small’, ‘little’ but can also mean ‘short’ – the question is, which meaning did Jesus have in mind?
There is in my view a straightforward, and important answer. The most suitable translation is, I believe, made clear from the teaching of Jesus that follows in Matthew 17. If ‘oligo’ means ‘small’ or ‘little’ it would not really make sense… why would Jesus tell his disciples that they were of ‘little faith’ and then tell them they only needed a small amount of faith – faith as a mustard seed? (although the linguistic irony may have been deliberate). However, if Jesus was not talking about the size of faith, so much as the length of faith, and ‘oligopistian’ means ‘short faith’ the text begins to make sense – in fact, this is how the term is ‘oligon’ is translated in Revelation 12:12 to speak of a short amount of time, ὀλίγον καιρὸν (oligon kairon). With this in mind, Jesus would have been right to have labelled Peter as having ‘short faith’ by Jesus, Peter certainly had faith when he stepped out of the boat... but he was unable to maintain his faith in view of the wind and the waves! Similarly, Jesus’ disciples, it seems, gave up too soon in their ministry to the troubled boy. Mustard seed faith, then, is the type of faith that endures, it continues, it grows until the answer arrives – Jesus’ teaching, then, appears to be an encouragement to ‘always pray and not give up’ (Luke 18:1) and our faith, should not be short-lived but long and enduring in the face of adversity and challenge!