This means the oneness of the world of humanity.

The most important thing is to polish the mirrors of hearts in order that they may become illumined and receptive of the divine light. One heart may possess the capacity of the polished mirror; another be covered and obscured by the dust and dross of this world. Although the same Sun is shining upon both, in the mirror which is polished, pure and sanctified you may behold the Sun in all its fullness, glory and power revealing its majesty and effulgence, but in the mirror which is rusted and obscured there is no capacity for reflection although so far as the Sun itself is concerned it is shining thereon and is neither lessened nor deprived. Therefore our duty lies in seeking to polish the mirrors of our hearts in order that we shall become reflectors of that light and recipients of the divine bounties which may be fully revealed through them.
This means the oneness of the world of humanity. That is to say, when this human body-politic reaches a state of absolute unity, the effulgence of the eternal Sun will make its fullest light and heat manifest. Therefore we must not make distinctions between individual members of the human family. We must not consider any soul as barren or deprived. Our duty lies in educating souls so that the Sun of the bestowals of God shall become resplendent in them, and this is possible through the power of the oneness of humanity. The more love is expressed among mankind and the stronger the power of unity, the greater will be this reflection and revelation, for the greatest bestowal of God is love. Love is the source of all the bestowals of God. Until love takes possession of the heart no other divine bounty can be revealed in it.

 Selected Writings of ‘Abdu’l-Bahá