From the inside front cover:
The varying titles by which the priest is addressed indicate that this is much more than a letter to an individual. It is intended for a larger audience -- a presentation of the Faith to humanity. Shoghi Effendi tells us, in God Passes By, that Baha’u’llah “quotes some of the most characteristic and celebrated passages of His own Writings” in this work, “and adduces proofs establishing the validity of His Cause.” Although it is an independent creative work, having its own unity of form, its own personal spirit – it is almost an anthology, and one selected by Baha’u’llah Himself. Its great impact on the reader is the eternal gift it holds out to him of the mercy of God. It helps us to enter His presence. It brings us to “Him Whom the World hath cast away and the nations abandoned.”
This book is the last earthly gift from Baha’u’llah. His enemies brought Him poison, but He changed it into honey for His loved ones.
Between these two so-called general letters - The Advent of Divine Justice and The Promised Day Is Come - Shoghi Effendi gave the western believers his fifth and last book of translations of the Writings of Bahá'u'lláh, undertaken during the winter of 1939-40, at another of the most difficult and hazardous periods of his life, and mailed to America for publication on the eve of his departure for Europe in the teeth of the European war. The Epistle to the Son of the Wolf was Bahá'u'lláh's last major work and contains a selection from His own Writings made by Himself (surely a unique occurrence in religious history!) during the last two years of His life and has therefore a special position of its own in the literature of our Faith. In a cable shortly prior to its publication Shoghi Effendi said "Devoutly hope its study may contribute further enlightenment deeper understanding verities on which effective prosecution teaching administrative undertakings ultimately depend..." (Ruhiyyih Khanum, The Priceless Pearl, p. 222)
This momentous Epistle was revealed by Bahá'u'lláh in the last year of His earthly life. It is addressed to Shaykh Muhammad-Taqi, known as Aqa Najafi, a son of Shaykh Muhammad-Baqir who was stigmatized by Him as 'Wolf'. After the death of his father in 1883, Aqa Najafi succeeded him as a leading mujtahid of Isfahan. He was an inveterate enemy and formidable opponent of the Cause of Bahá'u'lláh, a vicious and evil-minded clergyman who fulfilled the famous saying: 'The child is the secret of his sire.' He well merited the title 'Son of the Wolf'.
To such a man, who was perpetrating the most heinous crimes against His followers in Persia, Bahá'u'lláh addressed this weighty Epistle. The opening paragraph is in praise of God, and the second, in praise of Bahá'u'lláh as the 'Supreme Mediator', 'the Most Exalted Pen', 'the dawning-place' of God's 'most excellent names', and 'the dayspring of His most exalted attributes'. Having unequivocally announced His own station to Aqa Najafi, Bahá'u'lláh in the next paragraph proclaims to him that 'the ear of man hath been created that it may hearken unto the Divine Voice of this Day', counsels him first to 'purify' his soul 'with the waters of renunciation', to 'crown' his head with 'the crown of the fear of God' and then to arise from his seat, turn his face in the direction of Bahá'u'lláh's abode and recite a prayer which is revealed especially for him.” (Adib Taherzadeh, The Revelation of Baha'u'llah v 4, p. 368)