Parables are challenging to understand. After all, Jesus shared them a little over 2,015 years ago. It helps to put on First Century thinking caps in order to grasp their meaning. That said; let’s look at the parable about a grain of mustard seed.
Here the mustard seed represents something which must die when it is planted. It has been said the Kingdom of Heaven is the mustard seed. But consider the following: Lord Jesus ushered in the Kingdom of Heaven at His birth. John the Baptist’s ministry revealed it to the Jews. Then it spread when Jesus began His ministry since He told His disciples to preach about it so the end would come. The Kingdom of Heaven will never die, thus it cannot be the mustard seed.
Now consider this; one of Jesus’ names is the Word (John 1:1 & 14). Other Scriptures also reference Jesus and planting (Romans 6:4-5). Could the explanation of the parable be that the Word of God is the mustard seed? If so, then the man who sowed the seed in the parable is the Holy Spirit and the field is the hearts of men. When the Word is sown in the hearts of men by the Spirit, it grows into the Kingdom of Heaven in their hearts. Thus it replaces the kingdom of darkness.
The actions of the enemy against the Word being sown in people’s hearts support this. “And these are they by the way side, where the word is sown; but when they have heard, Satan cometh immediately, and taketh away the word that was sown in their hearts. (Mark 4:15)”
And the birds of the air are the souls of the redeemed. First Century Jews understood that the birds meant their souls because of this line from the Psalms. “Our soul is escaped as a bird out of the snare of the fowlers: the snare is broken, and we are escaped. (Psalm 124:7)”
Jesus shared another parable about a corn of wheat falling into the earth and dying just before His death (John 12:23-24). He could have said the Kingdom of Heaven was like a corn of wheat in this parable. However, the size of the mustard seed is the point He is emphasizing. It is the image of something starting out small, yet becoming the greatest, which is being taught. As believers share God’s Word, the Kingdom of Heaven grows.
This is what the Kingdom of Heaven is like. The Word/Jesus died and was resurrected. Lord Jesus’ death ensured the Kingdom of Heaven could grow and become the greatest kingdom in the hearts of men. Now, His Spirit sows the Word in their hearts so the royal children who have been redeemed out of the kingdom of darkness can find shelter in the Kingdom of Heaven.
(Excerpt from The Royalty Principle: a Guide to Reigning in the Kingdom of Heaven)