“The kingdom of heaven is like a mustard seed..."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 consciousness. The consciousness relating to our relationship with the Supreme Being.
Jesus is comparing the development of a loving service relationship between the living being and the Supreme Being - the foundation of the kingdom of heaven - to a seed that grows into a plant and eventually a tree which can offer shelter to others.
This is because love for God has a humble beginning, and starts with a simple introduction by God's representative - an already accomplished loving servant of God with an existing loving relationship with the Supreme Being.
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.
"Though it is the smallest of all your 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 the heart of the student as the student learns from God's representative. God's representative also illustrates the loving service relationship with the Supreme Being through their own service to the Supreme Being.
This is precisely what Jesus did with respect to his disciples and students. 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.
"...yet when it grows, it is the largest of garden plants and becomes a tree..."Following the introduction to the Supreme Being, the student begins the practice of worshiping and serving the Supreme Being under the guidance of God's representative. Gradually, the student begins to develop their own relationship with the Supreme Being. This is the process of growth that Jesus discusses here.
"...so that the birds of the air come and perch in its branches.”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.
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 being caring 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 are lonely and empty without our innate loving service relationship with the Supreme Being.
The original seed given by God's representative eventually spouts into a plant within the heart of the student, and then - if properly watered and protected - 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.
(For a translation of Jesus' statements from the Gospel of Matthew without institutional sectarian influence, see the Devotional Translation - translated from the original Greek texts.)