The third and last parable that Mark recorded Jesus giving to the multitudes stressed the contrast between the kingdom"s insignificant beginnings and its final impressively large size. When Jesus came declaring that the kingdom of heaven was at hand, He began preparations for the inauguration of the kingdom. He planted the seed. That beginning was a very inauspicious one. Even though Jesus had a popular following, He had few disciples who followed Him faithfully. Nonetheless this parable assured the multitudes that the kingdom would one day be impressively large. The Old Testament predicted that it would cover the earth and incorporate Gentiles as well as Jews ( Psalm 2; Ezekiel 17:22-24; Ezekiel 31:6; Daniel 4:12; et al.). The final form of the kingdom is at the end of the kingdom, not at its beginning when Jesus comes at His second coming to begin it. The parable describes the kingdom, not the church (all genuine Christians) and not Christendom (all professing Christians). [source][source][source]
The beginnings of the kingdom were small and discouraging. Jesus experienced rejection and left this world as an apparent failure. Nevertheless God will eventually establish the kingdom that the Old Testament prophets and Jesus predicted as a worldwide organization that will dominate all aspects of life. This hope encourages believers, especially believers who are suffering for their faith. We can press on knowing that our labor in spreading the gospel is not in vain. [source][source][source]
"The example of the mustard seed should prevent us from judging the significance of results by the size of the beginnings." [1][source]