O ye, of little faith
This is the rebuke levelled at the disciples of Christ, when seeming to doubt his divinity. The phrase is also more widely used to describe any Christian doubter. In a secular setting it may be intended as a humorous jibe when doubting someone's abilities.
There are several places in the Bible in which this phrase is used with reference to those who demonstrate their lack of faith in Jesus' power to perform miracles. Here is an example from Miles Coverdale's Bible, 1535:
Luke 12:27 Considre the lilies vpo the felde, how they growe: they laboure not, they spynne not. But I saye vnto you: that euen Salomen in all his royalte was not clothed like one of these.
Luke 12:28 Wherfore yf God so cloth the grasse, yt is to daye in ye felde, and tomorow shalbe cast in to the fornace, how moch more shal he clothe you, o ye of litle faith?
In the 17th century, the people that we would now call atheists were called nullfidians. The state of insufficient faith was also of common enough interest to be given a name - petty fidianism. John Trapp, in his Commentary on the Gospel of Matthew, 1647, recorded the term:
"O ye of little faith. Ye petty fidians; He calleth them not nullifidians."