“The kingdom of heaven is like a mustard seed, which a man took and planted in his field. Though it is the smallest of all your seeds, yet when it grows, it is the largest of garden plants and becomes a tree, so that the birds of the air come and perch in its branches.” (Matthew 13:31-32)
Why a 'mustard seed'?Jesus was likely referring to the black mustard seed here. While the yellow mustard seed will typically only produce a small bush-like plant, the black mustard seed will produce a tree. This tree - translated from the Greek σίναπι (sinapi) - was cultivated in the Middle East for centuries - Brassica nigra (black mustard). The seeds were used as a spice - called senafitch. The seeds were also considered medicinal.
These plants also grew profusely, even in the dryest regions. They are hardy and drought-tolerant.
This is why Jesus utilized the mustard plant in his analogy. He is speaking of something that will grow profusely within our hearts if we let it, and will readily grow even if there are weeds that surround. It will also grow into a large, strong tree.
Furthermore, this mustard tree will eventually become large enough to provide shelter to others who want to find shade from the sun. This completes the analogy because someone who lets the seed of love for God develop within their hearts will at some point be able to offer aid to others who need shelter from the challenges of the material world.
What does this have to do with the kingdom of heaven?Jesus is comparing the development of a loving service relationship between the living being and the Supreme Being - which is the foundation of the kingdom of heaven.
The word "kingdom" is translated from the Greek word βασιλεία (basileia), which means, according to the lexicon, "royal power, kingship, dominion, rule; not to be confused with an actual kingdom but rather the right or authority to rule over a kingdom."
Jesus is not referring to a physical place here. He is referring to a particular consciousness: The consciousness relating to our relationship with the Supreme Being.
This is because love for God has a humble beginning, and starts with a simple introduction by God's representative. Here we see that Jesus is representing the Supreme Being as he taught his students.
What is the meaning of being the 'smallest' of seeds?There are two aspects of the concept of the small seed to be compared here. One is the fact that the introduction to the Personality of the Supreme Being simply requires a humble process of learning. It is not a grand ceremony. While the baptism or initiation process is often considered ceremonial and symbolic, the real introduction to the Supreme Being takes place within our heart as we begin to open up our eternal relationship with the Supreme Being.
This is precisely what Jesus asked his disciples and students to do. He taught them directly through words, and set an example by his actions.
He then instructed them to follow in his footsteps and go out and teach others what he taught them. This is the process of passing on the teachings of love for God.
What is the 'largest of garden plants'?
Jesus' metaphor, the "largest of garden plants" refers to accomplishing the greatest task within the physical world - coming to know and love the Supreme Being.
Love of God is the "largest of garden plants" because it accomplishes everything our heart is seeking. Love of God is fulfilling to us, and it also yields benefits for anyone that comes into contact, because love of God is infectious.
Following the introduction to the Supreme Being, we can begin the practice of sincerely worshiping and serving the Supreme Being, following in Jesus' footsteps. Gradually, this allows us to develop our own relationship with the Supreme Being. This is the process of growth that Jesus discusses here.
What does it mean that 'birds come and perch'?Naturally, as a person’s love for the Supreme Being grows, they also begin caring and loving others - the other children of the Supreme Being - our brothers and sisters.
Loving God and loving others provides shelter for others. This means others seek the shade and protection of having a loving relationship with God.
Because of the love the Supreme Being has for each of us, a person whose innate love for the Supreme Being begins to blossom will automatically begin to care about the other children of the Supreme Being.
They begin to care for the Supreme Being and what pleases the Supreme Being. As this care increases, the tree of loving service to the Supreme Being may cause the Supreme Being to empower that person to teach to others.
This teaching provides shelter to others because each of us is lonely and empty without our innate loving service relationship with the Supreme Being.
The original seed given by God and His messenger eventually sprouts into a plant within the heart of the student. Then - if properly watered and protected - this seed grows into the tree of love for God. This tree eventually offers fruit and shelter for others, because only the Supreme Being can truly give us shelter.