September 1. On this date in 1935, Shoghi Effendi wrote "Bahá'u'lláh's message is thus the only key to a true understanding of the mysteries that envelop man's spiritual life."

September 1. On this date in 1935, Shoghi Effendi wrote "Bahá'u'lláh's message is thus the only key to a true understanding of the mysteries that envelop man's spiritual life."

With regard to your question concerning the meaning of the name "Hidden Words": It is, indeed, one of the most suggestive titles of the Writings of Bahá'u'lláh. These words are called "hidden" due to the fact that men have had neither the knowledge nor a true sense of appreciation of them before they were revealed by Bahá'u'lláh. It is through Him, Who is the sole Mouthpiece of God in this age, that spiritual realities and truths have been once more reinterpreted and revealed afresh to mankind. Bahá'u'lláh's message is thus the only key to a true understanding of the mysteries that envelop man's spiritual life.
(From a letter dated 1 September 1935 written on behalf of Shoghi Effendi to an individual believer)

The Hidden Words was written around 1857, after Bahá'u'lláh's two year study with Sufi sheikhs. Bahá'u'lláh had left Baghdad for the mountains of Kurdistan on April 10, 1854, and returned to Baghdad on March 19, 1856. He spent those two years using the name Darvish Muhammad-i-Irani studying with various Sufi sheikhs. His studies with the Sufis led to his writing the Four Valleys in 1857 and Seven Valleys in 1860. Both books are usually published together and their contents are largely based on the experiences he had as Darvish Muhammad-i-Irani. Similarly, the Kitáb-i-Íqán, written in 1861, contains many themes common to Sufi teaching. Finally, Bahá'í cosmology is largely a reflection of Sufi cosmology.