Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and become a tree, so that the birds of the air come and lodge in the branches thereof. Mat. 13:31-32 KJV
The kingdom of Heaven is compared to a mustard seed, that has a small beginning and grows rapidly to an abnormal size so that it becomes a tree sheltering things that it was not intended to shelter. The mustard seed is mentioned in Mt. 13:31; 17:20; Mk. 4:31; Lk. 13:19; 17:6. In all these passages the minuteness of the seed is referred to, while in the first three the large size of the herb growing from it is mentioned. Several varieties of mustard have notably small seeds and under favorable conditions grow in a few months into tall herbs – ten to twelve feet high. The rapid growth is always a striking fact. Among the rabbis a grain of mustard was a common expression for anything minute, a meaning which explains our Lord’s phrase, “faith as a grain of mustard seed”. Mt. 17:20; Lk. 17:6
The “birds” are the same that devoured the seeds by the wayside in the parable of the sower. They are demon powers headed by the devil. This parable illustrates rapid and abnormal growth of the kingdom of Heaven from an insignificant beginning to a great place in the Earth, from being poor, little known, and separate from the world to being a great worldly institution, and the habitation of wicked men and demon powers. Mt. 13:4, 19, 32; Rev. 17:1-18; 18:2 This parable emphasizes the truth of the parable of the tares and wheat – that God’s purpose was to have only wheat in the field and that the kingdom of Heaven should be made up of only good people, but an enemy sowed tares and changed the true aspect of the kingdom to an abnormal condition of a mixture of good and bad.