Compare Mat 21:21. Not equivalent to unbelief, but expressing the hesitation which balances between faith and unbelief, and inclines toward the latter. This idea is brought out in the next sentence.
A wave (κλύδωνι)
Rev., surge. Only here and Luk 8:24; though the kindred verb occurs at Eph 4:14. The word is admirably chosen, as by a writer who lived near the sea and was familiar with its aspects. The general distinction between this and the more common κῦμα, wave, is that κλύδων describes the long ridges of water as they are propelled in horizontal lines over the vast surface of the sea; while κῦμα denotes the pointed masses which toss themselves up from these under the action of the wind. Hence the word κλύδων here is explained, and the picture completed by what follows: a billow or surge, driven by the wind in lines, and tossed into waves. Both here and in the passage in Luke the word is used in connection with the wind. It emphasizes the idea of extension, while the other word throws forward the idea of concentrating into a crest at a given point. Hence, in the figure, the emphasis falls on the tossing; not only moving before the impulse of the wind, but not even moving in regular lines; tossed into rising and falling peaks.
Driven by the wind (ἀνεμιζομένῳ)
Only here in New Testament.
Only here in New Testament. From ῥιπίς, a fan. Anyone who has watched the great ocean-swell throwing itself up into pointed waves, the tops of which are caught by the wind and fanned off into spray, will appreciate the vividness of the figure.