From the same chapter...
"26... I said concerning faith—that it was not a perfect knowledge—even so it is with my words. Ye cannot know of their surety at first, unto perfection, any more than faith is a perfect knowledge.
27 But behold, if ye will awake and arouse your faculties, even to an experiment upon my words, and exercise a particle of faith, yea, even if ye can no more than desire to believe, let this desire work in you, even until ye believe in a manner that ye can give place for a portion of my words.
28 Now, we will compare the word unto a seed..."