In the Bahá'í teachings it states that all the Prophets have foretold a Promised One Who is Bahá'u'lláh. We cannot be sure of the authenticity, word for word, of any of the past Holy Scriptures except the Qur'án, as they were either not written down during the Prophet's lifetime or have been changed in the course of time and the originals lost; what we can be sure of is that when Bahá'u'lláh or the Master stated that Zoroaster foretold a Promised One's coming, it is correct. The Zoroastrians have no way of contradicting this assertion of ours, as they themselves know their scriptures are not in the original form, and therefore not absolutely authentic. (From a letter dated 22 June 1943 written on behalf of Shoghi Effendi to an individual believer; The Compilation of Compilations, vol. I, Extracts from the Baha’i Writings on Buddha, Krishna, Zoroaster, and related subjects)
