Luke 13:18 Then said he, Unto what is the kingdom of God like? and whereunto shall I resemble it?
The Spiritual kingdom of God is where God is with His Spirit. Remember what Jesus said earlier:
Luke 11:20 But if I with the finger of God cast out devils, no doubt the kingdom of God is come upon you.
Jesus was being accused by the religious leaders of doing miracles by the power of the devil and also healing on the sabbath. These were the rulers of the Jewish houses of worship. With this in mind Jesus gives these parables about the earthly places that claimed to represent the kingdom of God.
Luke 13:19 It is like a grain of mustard seed, which a man took, and cast into his garden; and it grew, and waxed a great tree; and the fowls of the air lodged in the branches of it.
First, a man planted a seed and it grew. Mustard is a bush, not a tree, but this mustard grew abnormally large. It had become something abnormal, perhaps in reference to huge churches. And, the fowls of the air lodged in it. The parable of the sower, described fowls as eating up the word, this was the devil or his demonic presence from the air in these earthly places representing God's kingdom. The devil has been described of the prince of the power of the air. False teaching and false doctrine in these big religious places.
Eph 2:2 Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:
Luke 13:20 And again he said, Whereunto shall I liken the kingdom of God?
Luke 13:21 It is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened.
Again in reference to that which represents the kingdom on earth, the synagoge or church that is full of leaven. Leaven is the representation of sin.
Mat 16:6 Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees.
Three measures of grain, was the grain offering. This was an offering as worship. Their worship, our worship, is corrupted by sin.
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.