polish the mirrors of hearts in order...

The light of the sun becomes apparent in each object according to the capacity of that object. The difference is simply one of degree and receptivity. The stone would be a recipient only to a limited extent; another created thing might be as a mirror wherein the sun is fully reflected; but the same light shines upon both.
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.

Selected Writings of Abdu’l-Bahá