The two parables that follow (Lk 13: 18-19, 20-21), form a pair, Luke’s source is ‘Q’ (Cf. Mt 13: 31-33). We have already seen the Markan form of the parable of the mustard seed (Mk 4: 30-32).
Coming as they do immediately after the preceding section on Jesus’ appeal to repentance and reform and the healing of the sick on the sabbath, these parables situate such activities in the broader context of his Kingdom-preaching.
The parable of the mustard seed is meant to show that the Kingdom of God certainly grows. Though its beginnings are small like that of the proverbially tiny mustard seed, the Kingdom is bound to grow into something great. Jesus’ own ministry of proclaiming the Kingdom, though mall in its beginnings, is certain to have a great end result. Luke does not make this contrast as explicit as Mark and Matthew do. His emphasis is on the process of growth of the seed, which, it is to be understood, is due to the hidden power of God at work in the seed itself (Cf. Ezek 17: 22-24; 31: 2-9). The Kingdom will have phenomenal growth; human beings, like birds, will find shelter in the big tree of God’s Kingdom. The certainty of its phenomenal growth is the focus of the parable.
Jesus’ mission of proclaiming the Kingdom, even though it is apparently insignificant and small in its beginnings and meets with misunderstanding and opposition will have inevitable result. His present mission and the end result are essentially related as seed with the tree. Jesus understood the Kingdom as a present reality; people are challenged to take shelter in it.