But with the love of God every bitterness is changed into sweetness and every gift becometh precious.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 366)
In a similar way, thou beholdest some women who have abandoned the Testament, and to them the bitterness of discord is sweet. They keep aloof from the Extended Shadow and dwell under the shade of a “black smoke.” Alas for them and grief for them! They will surely lament and find themselves in loss. Verily, this is but an evident truth!
(‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v1, p. 130)
O thou who art partaking of the Heavenly Food! Know thou verily the Divine Food is descending from heaven, but only those taste thereof who are directed to the light of guidance, and only those can enjoy it who are endowed with a sound taste. Otherwise every diseased soul disliketh the delicious and merciful food and this is because of the sickness which hath seized him, whereby the El-Zekkum  is sweet (to his taste) while he fleeth from the ripe fruit of the Tree of the Living and Pre-existent God—and there is no wonder in that.
(‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v1, p. 130-131)
[1 El-Zekkum—a thorny tree so called, which bears fruit like an almond, but extremely bitter. Therefore the tree symbolizes a very severe punishment and bitter remorse for the unbelievers.]
The Lord, peerless is He, hath made woman and man to abide with each other in the closest companionship, and to be even as a single soul. They are two helpmates, two intimate friends, who should be concerned about the welfare of each other. “If they live thus, they will pass through this world with perfect contentment, bliss, and peace of heart, and become the object of Divine grace and favour in the Kingdom of heaven. But if they do other than this, they will live out their lives in great bitterness, longing at every moment for death, and will be shamefaced in the heavenly realm. “Strive, then, to abide, heart and soul, with each other as two doves in the nest, for this is to be blessed in both worlds.”
(‘Abdu’l-Bahá, Lights of Guidance, p. 226)
The N.S.A.s the world over, owing to the spiritual immaturity of the believers, must at the present time exert the greatest patience in dealing with the friends; otherwise, as seems to be rapidly becoming the case in Australia, the friends will take sides, bitterness will increase and what started out as a small thing, (however unjustified and regrettable a departure from the Bahá’í spirit) will become a menace to the progress of the Faith and definitely retard its progress.
(Shoghi Effendi, Letters from the Guardian to Australia and New Zealand, p. 57)
To nurse a grievance or hatred against another soul is spiritually poisonous to the soul which nurses it, but to strive to see another person as a child of God and, however heinous his deeds, to attempt to overlook his sins for the sake of God, removes bitterness from the soul and both ennobles and strengthens it.
(Universal House of Justice to an individual believer, 5 January 1992)
When our thoughts are filled with the bitterness of this world, let us turn our eyes to the sweetness of God’s compassion and He will send us heavenly calm!
(‘Abdu’l-Bahá, Paris Talks, p. 111)