“… Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you” (Matthew 17:20).
Mustard plants are widespread throughout the world, often being cultivated to use as a spice, so the analogy Christ used was readily understandable to His hearers. So how small are the seeds? The mustard plant mentioned in the New Testament is probably the white mustard (Sinapis hirta) which bears quite small seeds usually about 1 to 3 mm (1/8 inch) in diameter.
The size of the mustard seed is often a subject of comment by sceptics because of Christ’s words recorded in Mark: “[The Kingdom of God] is like a mustard seed, which is the smallest of all seeds on earth. Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds can perch in its shade” (Mark 4:31-32). Some have been quick to point out, relative to the statement “which is the smallest of all seeds on earth,” that many plants have much smaller seeds. The seeds of some orchids, for example, are not much bigger than sand particles. But Christ’s words specify seeds which are “planted” – “garden plants” – so it is clear He was talking of plants commonly cultivated at that time. Additionally, the Greek simply states that they are the smallest seeds “on the ground” (meaning in the garden rather than on the surface of the whole earth), so there is no real problem here.
More important is the way Jesus uses the tiny mustard seed in a number of parables and sayings (Matthew 13:31–32, Matthew 17:20–21, Mark 4:30–32, Luke 13:18–19, Luke 17:6): both as a simile for the Kingdom of God that starts small, but grows great, and also in statements that if we have a mustard seed amount of faith we can accomplish great things. The two statements are perhaps related, because the instances where Jesus used the Kingdom parables of the mustard seed which is small but grows great are recorded as being given before the comments on the mustard seed representing a small amount of faith that can do great works. So it is likely that the “tiny mustard seed that grows” background was clear to the disciples, and that the reference to the amount of faith like a mustard seed was understood as faith that starts small but grows great. In fact, in Matthew 17:20, while NIV says “faith as small as a mustard seed,” the Greek is literally “faith like a mustard seed” (so it can mean like a mustard seed in its growth) and is so translated by KJV, ESV, NKJV, RSV, and most other versions.
This understanding fits much better with the fact that Jesus often chastised the disciples as being of “little faith” when they failed spiritually (for example, Matthew 8:26). This shows that a small amount of faith is not all that is necessary to accomplish great things. It is more likely then, that Jesus' parable of the mustard seed of faith relates not to its initial size, but rather to what can be accomplished if our faith grows, as the tiny mustard seed does, to a great size.
In either case, whether Christ meant we need a very small amount of faith, or a faith that starts small and becomes great, the message contained in the analogy He used remains the same – with believing faith we can accomplish great things. And as we "grow" our faith, we will be able to accomplish yet more!