Jesus selected familiar scenes to describe eternal truth. It should not surprise us that God's creation will remind the observant of their Creator, and that the Creator should use what He has made to illustrate eternal truth: although the crowds did not understand, and the disciples were very slow to grasp the truth (John 3:12).  On this occasion, having just told the parable of the sower, the seed and the soils (Mark 4:1-20), and another about the inherent life of a seed (Mark 4:26-29), Jesus describes the nature and purpose of the seed of the gospel.  It is all about the kingdom of God.  

Jesus is the King and His kingdom is the domain over which He has authority.  That is everywhere.  But the devil has grabbed God's right to rule to be the 'god of this world' (2 Corinthians 4:4).  That is why Jesus challenged Satan's authority in the wilderness (Matthew 4:1-11).  It is also why the gospel is preached to rescue us from this evil dominion (Galatians 1:3-4), and from the demonically inspired fear of death (Hebrews 2:14-15).  All who accept His authority become citizens of His kingdom.

In this story, Jesus chooses the mustard seed to illustrate His point.  That seed is so very small that it is hardly noticeable when dropped onto soil.   But it has potential to grow massively to become a place of shelter for birds who might have eaten the seed, a refuge from the weather, a place to stay and rest (Psalm 91:1).  The unnoticeable seed has become an all-embracing sanctuary (Isaiah 4:5-6).  What a difference when seed is allowed to grow where it is planted!

The effect of God's Word is quite disproportionate when 'planted' in a receptive heart (Mark 4:20).  As it grows, it envelops the heart which contains it.  The kingdom of God becomes much bigger than one individual, or village or city or country.  As it unfolds and grows, it becomes a massive place of safety ... even for those who, like birds eating the seed, would previously have used it for short term gain.  We can think of unbelievers seeking churches for their weddings and funerals; taking a preacher's time to sign passport applications or school references; or gladly receiving what the church can do to supply their personal needs.  Little do they know the huge change that would come if they received the gospel instead of using the church.  But you, reading this, probably do know.  That is why you have been given the privilege of holding the seed in your hands!  It is to be planted in people's hearts.  So, plant the seed, and let it grow, and grow and grow.