Unbelief. Textual evidence is divided (cf. p. 146) between this reading and “little faith.” The disciples had too much faith in themselves and too little faith in God (see on ch. 8:26).
Mustard seed. See on ch. 13:31, 32. For similar illustrations given upon other occasions see Matt. 21:21; Mark 11:23; Luke 17:6. The mustard seed maybe small to begin with, but hidden within it is the life principle, and, given favorable circumstances, it will grow.
Say unto this mountain. Christ here speaks figuratively of great obstacles encountered by His disciples as they carry out the gospel commission. Most assuredly Jesus never intended that His disciples should go about the country moving literal mountains. Nevertheless He promised that no difficulties, however great they may seem, can hinder the accomplishment of His divine purpose to save sinners (see Isa. 45:18; 55:8-11).
Nothing shall be impossible. “With God all things are possible” (Matt. 19:26).