What does the mustard seed symbolize?

Study a plant of mustard—witness it grow and bud. Study the truth of mustard life.

Only the strong in God can act upon this teaching and hold the lower nature. The weak must wait its maturity, and then will come their struggle. It will strive to keep the feet from the path—and may succeed. But if once all its power be wiped out—if once thou doest the will of the Father earnestly—is His obedient child—that is the atonement, for it shall give strength to do every work of the Creator of Being. It will seem to take the very life. That is because it takes the animal soul and throttles it. But the human soul will recover, and the spirit come into it. 

This is the time of the Silence of the Soul.

Then it shall be clear to you how dark are the lives of those who are around you and have no goal of union with the spirit towards which to race.

And you will see and know Karma.

Also, you will see that because of your past incarnations, your Karma is inextricably interwoven with the Karma of the world. This is that saying, which the Nazarene answered when it was asked of Him—

Who is my neighbour?

If your soul once knows this peace, no storm of sin or sorrow can ever more ward you far aside from the Path, for its knowledge is an abiding wisdom.