Be thou not sad and dispirited on account of the opposition of the heedless ones. Soon they shall regret sorely. Reflect thou how the Pharisees persecuted and looked down in contempt upon His Holiness the Christ. The result was that His lamp became ignited, His light began to shine and His followers sparkled like unto the stars from the horizon of existence; and the consequence to the Pharisees was the pangs of remorse and regrets.

‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v1, p. 221

But this wronged one, this prisoner, did his utmost to hide and veil their doings, that haply they might regret and repent.

‘Abdu’l-Bahá, The Will and Testament, p. 24

Even this earth's happiness does not depend upon wealth. You will find many of the wealthy exposed to dangers and troubled by difficulties, and in their last moments upon the bed of death there remains the regret that they must be separated from that to which their hearts are so attached. They come into this world naked, and they must go from it naked. All they possess they must leave behind and pass away solitary, alone. Often at the time of death their souls are filled with remorse; and worst of all, their hope in the mercy of God is less than ours.

‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 33

My heart aches, for I note with intense regret that the attention of the people is nowhere directed toward that which is worthy of this day and time.

‘Abdu’l-Bahá, The Secret of Divine Civilization, p. 52

Notwithstanding this great Covenant, a few negligent souls galloped with their chargers into the battlefield, thinking perchance they might be able to weaken the foundation of the Cause of God: but praise be to God, all of them were afflicted with regret and loss, and ere long they shall see themselves in poignant despair. Therefore, in the beginning one must make his steps firm in the Covenant -- so that the confirmations of Bahá’u’lláh may encircle from all sides, the cohorts of the Supreme Concourse may become the supporters and the helpers, and the exhortations and advices of ‘Abdu’l-Bahá, like unto the pictures engraved on stone, may remain permanent and ineffaceable in the tablets of the hearts.

‘Abdu’l-Bahá, Bahá’í World Faith, p. 425-426

Soon the lights will glisten and the Sun of Truth will shine with a burning ray in that country. At that time the maid-servants who are firm and constant will rejoice; while every doubtful soul who wavers will regret. Woe unto him, woe unto him, in that day wherein the beloved will attain joy by the surrounding Kingdom of thy Lord, the Precious, the Beneficent!

‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v1, p. 96

What result is forthcoming from material rest, tranquillity, luxury and attachment to this corporeal world! It is evident that the man who pursues these things will in the end become afflicted with regret and loss.

‘Abdu’l-Bahá, Tablets of the Divine Plan, p. 42