Mark 4:30-32
30 Again he said, “What shall we say the kingdom of God is like, or what parable shall we use to describe it? 31 It is like a mustard seed, which is the smallest seed you plant in the ground. 32 Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds of the air can perch in its shade.”
Some non-believers say that the mustard seed is not the smallest seed you plant and Jesus is not right in giving us the Parable of the Mustard seed, because there are smaller seeds...These people miss the whole lesson in the Parable of the Mustard Seed...And it is very possible that the mustard seed was the smallest plant seed at the time of Jesus...His listeners would have known what He meant...
The Kingdom of God was small at the time of Jesus...The Kingdom of God would grow in numbers, and the gospel of Jesus, like the mustard seed, was small in number of believers...Over the next two thousand years the seeds of the gospel would grow, into something very, very large...The gospel would be taken to all the nations of the earth and become larger and larger...