Matt 13:31,32; Mk 4:30-32;
Lk 13:18,19 Then Jesus asked, "What is the kingdom of God like? What shall I compare it to? It is like a mustard seed, which a man took and planted in his garden. It grew and became a tree, and the birds of the air perched in its branches." (NIV)
A mustard seed is in the category of the smallest of all seeds in the area. Jesus was most likely referring to the black mustard, which was grown for its production of oil. They may grow up to fifteen feet and have a thick main stem with branches strong enough to bear the weight of a bird.
Jesus had elsewhere used the mustard seed in his parables for its small size.
Lu 17:6 He replied, "If you have faith as small as a mustard seed, you can say to this mulberry tree, 'Be uprooted and planted in the sea,' and it will obey you."
Therefore one inference one can make from this parable is that Christianity - the body of Christ - grew from a small size to large. The point being that one should not make too much of the size of a movement to validate it. For there are movements that are popular, but wrong. And there are movements which are unpopular which are correct. Also one of the themes of the Bible is that those who are truly God's people are often described as a "remnant". That is, they compose a minority of those who are nominally considered God's people. The apostle Paul even describes himself that way in Romans 11:1-5. We also notice that the Bible often focuses on individuals - Noah, Abraham, Moses, David, etc. In fact most of the books of the Bible are named after individuals. And God promises his people:
"The least of you will become a thousand, the smallest a mighty nation. I am the LORD; in its time I will do this swiftly." Isa 60:22
This is another parable on nominalism - the fact that the visible Church is mixture of true and nominal Christians.
When a movement becomes large and popular and institutionalized, it is inevitable to attract birds which will make nests in its branches. The birds are not part of the tree, but aliens to it. Jesus speaks of birds in the parable of the sower, equating them to the devil who takes away the word which is sown along the path.
The birds often nest high in the branches, taking an exalted position in the visible church, yet being foreign to. And there are those who try to build a nest egg from their association with the visible church. Even in his farewell speech speaking to the Ephesian elders, Paul warns:
"I know that after I leave, savage wolves will come in among you and will not spare the flock." Acts 20:29
These birds may also represent false ideas being integrated into Christian community. There are churches that think that there is safety in numbers. There isn't. Popular ideas are often contrary to the Biblical truth. And institutional philosophy has resulted in a great deal of corruption in the body of Christ. But this is generally true of any movement which becomes popular and institutionalized.
Therefore, let's not get too comfortable if Christiantity is popular, or if our particular institutional church becomes large. For things which are large and popular are much more subject to birds and their corrupting influence.