Atonement

Arise, and, under the eyes of God, atone for your failures in duty towards Him. This is My commandment unto you, were ye to incline your ears unto My commandment.

Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 293

He, in truth, hath offered up His life as a ransom for the redemption of the world.

Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 315

May my spirit be a sacrifice to the wrongs Thou didst suffer, and my soul be a ransom for the adversities Thou didst sustain.

Bahá’u’lláh, Prayers and Meditations by Bahá’u’lláh, p. 311

As to the question regarding the soul of a murderer, and what his punishment would be, the answer given was that the murderer must expiate his crime: that is, if they put the murderer to death, his death is his atonement for his crime, and following the death, God in His justice will impose no second penalty upon him, for divine justice would not allow this.

‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 179

As to the souls who are born into this world radiant entities and who through excessive difficulty are deprived of great benefits and thus leave the world -- they are worthy of all sympathy, for in reality this is worthy of regret. It is for this purpose (that is, it is with regard to this wisdom) that the great Manifestations (of God) unveil themselves in this world, bear every difficulty and ordeal -- to make these ready souls dawnings of light and confer upon them eternal life. This is the real atonement that His holiness Christ made-He sacrificed Himself for the life of the world.

‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v3, p. 542

In a highly significant prayer, revealed by Bahá’u’lláh in memory of His son -- a prayer that exalts his death to the rank of those great acts of atonement associated with Abraham's intended sacrifice of His son, with the crucifixion of Jesus Christ and the martyrdom of the Imam Husayn -- we read the following: "I have, O my Lord, offered up that which Thou hast given Me, that Thy servants may be quickened, and all that dwell on earth be united." And, likewise, these prophetic words, addressed to His martyred son: "Thou art the Trust of God and His Treasure in this Land. Erelong will God reveal through thee that which He hath desired.

Shoghi Effendi, God Passes By, p. 188