Jesus told the following two parables:
The Parable of the Mustard Seed
The kingdom of Heaven is like a mustard seed that a man took and sowed in his field. It was the smallest of all the seeds in Israel at that time, but when it had grown it was bigger than all the plants of the garden and became a tree, so that
the birds of the air came and built nests in its branches.
(Mat 13:31–32, Mark 4:31–32, Luk 13:18–19).
This parable isn't told just so that we understand that the kingdom of God will experience growth (Mark 4:26–29). When birds are used symbolically in the Bible, they often symbolize evil or unsaved people, see The Parable of the Sower.
This means that bad things would enter. Christianity became the state religion in
Roman Empire around 312 AD. Confessing Christianity became an advantage. God's kingdom was growing explosively, but it wasn't good growth. It was corrupt and political motivated. It led to The Roman Catholic Church. The so-called Dark Ages were between 500 and 1500 AD. It was a time when the church was filled with corruption and murder, often led by The Roman Catholic Church at the time. This is a historical fact. Around 1500 AD Martin Luther brought the Reformation.
The Parable of the Leaven
The kingdom of Heaven is also like leaven that a woman took and hid in three measures of flour, till it was all leavened (Mat 13:33, Luk 13:20–21).
1 Cor 5:6–7, Gal 5:9, Exo (2 Mos) 13:3–7). This parable is therefore negative.
The kingdom of Heaven is like this corrupting influence, it's going to look really bad. Even in the early decades of the early church we can see this permeation of corruption and sin in the church, see Ananias and Sapphira.
Summary
Jesus predicts this how the kingdom of Heaven will look like in the churches after Jesus' ascension into Heaven. There will be a separation in the church when Jesus returns, see The Parable of the Fish Net.
