The word ‘kingdom’ in a phrase like “the kingdom of God” is (wait for it) a metaphor. That is, a literary figure of speech that describes something by comparison. And, as usual, it’s not about the metaphor, it’s about the subject. In this case (Mark 4:26-34) it is God’s realm, where God is, that is being described.
From time to time people complain about this metaphor. Kingdom is masculine, hierarchical and militaristic. These are all qualities unintended by Jesus’ use of the metaphor. Neither is the afterlife the intended point. What is intended is, an organization of people, so Jesus could have said family (and does in places). It’s okay to challenge the metaphor but we mustn’t lose sight of the point.
The point is; how can this one metaphor (the kingdom of God) be compared to another metaphor so that the fullness of its meaning will be conveyed. Jesus says, it’s like scattering seed and it’s like mustard seed. On the one hand it is a common (annual at least) activity, that takes time to produce results. On the other hand, it can be a seemingly small activity that surprisingly produces grand results. Clearly, a masculine, hierarchical, militaristic or afterlife meaning is not intended.
We scatter the seeds of God’s realm, even if the seed seem the smallest of all seeds, a structure as big as a tree will grow, particularly amazing when we compare the size of the seed to the size of the tree. The tree is so big that even the birds of the air will come and live there. Need I even say it? Birds are a metaphor for God’s Holy Spirit, so even the Holy Spirit will come and live with us in God’s realm. And for us, God’s realm is this Church and is now - be patient.