In 1844, Tahirih was one of the Shaykhis who actively sought the Promised One and did not want to spend her time in theological speculation. The day of God was an imminent reality for her. It was for her brother-in-law as well. She gave him a letter written by her addressed to the Promised One. If he succeeded in finding Him he should give Him the letter.

One night in the summer of 1844, Tahirih had a dream: She saw a young Siyyid—a male descendant of the Prophet Muhammad— dressed in black with a green turban, and his hands raised up in prayer. The words he spoke stayed with her.

Around the same time, her brother-in-law met the Bab and became a believer and gave him Tahirih’s letter. The Bab immediately declared her to be one of his apostles. He gave her the title of “Letters of the Living.”

The Bab was the Primal Point from which all came into being, and His apostles were the Letters that originated from that Point. The Bab and the eighteen ‘Letters’ made nineteen, a ‘vahid’, or unity, signifying the unity of God; it is also the numerical value of the opening invocation of the Qur’an.

Though Tahirih never actually met the Bab in person, what confirmed her faith was reading a copy of a text revealed by Him. Tahirih recognized immediately that this commentary contained the very same words she had heard in her dream. She was now certain she had found the object of her spiritual search—the Promised One of the age as foretold by her Shaykhi spiritual teachers.

Click below for a superb documentary on the world of the Apostles of Jesus and read on for a further explanation of the theological Babi terms above.

“The term "Letters of the Living" is both a title and a theological statement. The expression comprises two Arabic words: hurúf (singular: harf), meaning "letters," and hayy, meaning "the living." The combination hurúf-i-hayy is new; it does not occur in the Islamic scriptures. In His early writings, the Báb also referred to His first disciples by the word sábiqún or sábiqín (the forerunners), which stems from Islamic Traditions and texts.
The term "letter" is symbolic, as is the Báb’s use of the term Nuqtih (Point) to refer to the Manifestation or Messenger of God, who is the embodiment of the Primal Will (a concept similar to the Logos or "Word" in Christianity). According to the Báb, God created the Primal Will through the causation of the Primal Will itself and then created all things through the causation of the Primal Will; in other words, the Creator of the cosmos and spiritual civilization is the Manifestation of God.”  (Source: Baha’i Encyclopedia Project)