As a grain of mustard-seed - See the notes at Matthew 13:31-32. The mustard-seed was the smallest of all seeds. It has been supposed by some, therefore, that he meant to say, If you have the smallest or feeblest faith that is genuine, you can do all things. The mustard-seed produced the largest of all herbs. It has been supposed by others, therefore, to mean, If you have increasing, expanding, enlarged faith, growing and strengthening from small beginnings, you can perform the most difficult undertaking. There is a principle of vitality in the grain of seed stretching forward to great results, which illustrates the nature of faith. Your faith should be like that. This is probably the true meaning.
Ye shall say unto this mountain ... - Probably he pointed to a mountain near, to assure them that if they had such faith they might accomplish the most difficult undertakings - things that at first would appear impossible.