How to “cleanse and purify” one’s heart and “adorn it with the spotless garment of holiness”

… love of God and spiritual attraction do cleanse and purify the human heart and dress and adorn it with the spotless garment of holiness; and once the heart is entirely attached to the Lord, and bound over to the Blessed Perfection, then will the grace of God be revealed. (Abdu'l-Baha, Selections from the Writings of Abdu'l-Baha, p. 202)

The “color" of one’s "servitude to God”

There are no whites and blacks before God. All colors are one, and that is the color of servitude to God. Scent and color are not important. The heart is important. If the heart is pure, white or black or any color makes no difference. God does not look at colors; He looks at the hearts. He whose heart is pure is better. He whose character is better is more pleasing. He who turns more to the Abha Kingdom is more advanced. ('Abdu'l-Baha, excerpt from a talk given at Howard University on 23 April 1912; The Promulgation of Universal Peace, pp. 44-45) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)

We can expect difficulties and hardships when attempting to serve the Cause of God

O maid-servant of God! Everything of importance in this world demands the close attention of its seeker. The one in pursuit of anything must undergo difficulties and hardships until the object in view is attained and the great success is obtained. This is the case of things pertaining to the world. How much higher is that which concerns the Supreme Concourse! That Cause involves every favor, glory and eternal bliss in the world of God. The seeker after the great guidance and eternal happiness necessarily will encounter difficulties. He must be patient under such circumstances. The chosen believers of the past quaffed the chalice of suffering and sank deep in the ocean of trials until they attained to that blessed station and sublime beatitude. (‘Abdu'l-Baha, Tablets of Abdu'l-Baha v2, p. 265)

The “light” of a human being

The light of man depends on the light of the love of God. (‘Abdu'l-Baha, Tablets of Abdu'l-Baha v2, p. 249)

“Do ye not look upon the beginning of the affairs; attach your hearts to the ends and results.”

Do ye not look upon the beginning of the affairs; attach your hearts to the ends and results. The present period is like unto the sowing time. Undoubtedly it is impregnated with perils and difficulties, but in the future many a harvest shall be gathered and benefits and results will become apparent. When one considers the issue and the end, exhaustless joy and happiness will dawn.
Live thou as far as thou art able according to the divine commands and advices, because they are conducive to eternal life. (‘Abdu'l-Baha, Tablets of Abdu'l-Baha v2, p. 264)

Spiritual vs. material light

The light is divided into two kinds, material and spiritual. The material light is a vanishing matter and is known by etheric vibrations. But the spiritual light is the divine, eternal and never-ending quality and is a truth of the Kingdom. Blessed is thy face for that it hath taken a portion and a radiance from that Light. (‘Abdu'l-Baha, Tablets of Abdu'l-Baha v2, p. 249)

How can one have an “effect upon the hearts” of others so they become attracted to “the Kingdom of God”

O maid-servant of God! Forget all else save God, be in communion with Him, supplicate and pray to Him to make thee conqueror over the material things, impressed by the bounties of the Kingdom, commemorating the name of thy Lord, pure from all else save Him, and imbued with the spiritual attributes of those who are holy (and high); then shall thy breaths have effect upon the hearts, and attract the hearts of thy son and husband unto the Kingdom of God. (‘Abdu'l-Baha, Tablets of Abdu'l-Baha v2, p. 246)

We should always "rely upon the confirmation of the Holy Spirit.“

O ye beloved of Baha'! Do not look at your weakness, nay, rely upon the confirmation of the Holy Spirit. Verily, It maketh the weak strong, the lowly mighty, the child grown, the infant mature and the small great. (‘Abdu'l-Baha, Tablets of ‘Abdu'l-Baha v2, p. 273)