Jesus, as He taught, used the mustard seed in two different analogies. We at times have mixed the two, causing us to miss at least one of the teachings.
In Matthew 13:31, Mark 4:31 & Luke 13:19 Jesus compares the Kingdom of God (the Kingdom of Heaven) to a mustard seed growing from very small to very large. This analogy focuses on the size of the seed initially and the plant it becomes. The message to the disciples is that the Kingdom thought it will start out small, will grow.
The other mustard seed analogy He uses is found in Matthew 17:20 & Luke 17:6. In this analogy He is not speaking of the size of the mustard seed, but the faith of the mustard seed. The mustard seed responds to the word of God. The mustard seed doesn’t doubt it’s Creator. It’s faith is pure. Mankind is created to reason and choose; therefore he can doubt the word of God. It’s when God speaks to us & we believe without doubting, that He performs His word to us.
Why couldn’t the disciples cast out the demon in Matthew 17:15-16? Jesus said that it was because of there unbelief. They had been given the commission to heal the sick and to cast out devils (Matthew 10:1). They had been doing so (Luke 10:17). But now unbelief had creep in. Why? Maybe it was the antics of the young boy that had caused them to doubt. We are not told. But we are told how to purify our faith, so that we can do that which God has told us to do. Unbelief is dispelled by prayer and fasting.
In Luke 17:5 we read that the apostles ask Jesus to increase their faith. His answer in verse 6 is meant to teach them that the amount of faith wasn’t the problem. It’s mustard seed faith – pure faith, faith without doubt – that they needed, as do we.
Romans 10:17 says, “Faith comes by hearing, and hearing by the word of God.” The more we hear from God the larger our faith becomes. Large faith, mixed with doubt, accomplishes nothing. Hear God. Believe God. Don’t doubt. And then you can do that which He has commissioned. If you find yourself doubting, pray and fast. Your faith will be purified and you will be able to serve in the way that He has called you to serve.