According to Bahai, Moses and Jesus and Allah were also Messengers. Since you disregard the OT and the NT and probably the Koran, how did these Messengers communicate God's teachings to man?
Clearly, the Baha'i Faith does not disregard the OT, the NT, or the Koran.
These are the sacred scriptures whereby these Messengers communicated God's teachings to man.
From Letters Written on Behalf of the Guardian:
...The Bible is not
wholly authentic, and in this respect is not to be compared with the Qur'an, and should be wholly subordinated to the authentic writings of Bahá'u'lláh.
(28 July 1936 to a National Spiritual Assembly)
...we cannot be sure how much or how little of the four Gospels are accurate and include the words of Christ and His undiluted teachings, all we can be sure of, as Bahá'ís, is that what has been quoted by Bahá'u'lláh and the Master must be absolutely authentic. As many times passages in the Gospel of St. John are quoted we may assume that it is his Gospel and much of it accurate.
(23 January 1944 to an individual believer)
When 'Abdu'l-Bahá states we believe what is in the Bible, He means in substance. Not that we believe every word of it to be taken literally or that every word is the authentic saying of the Prophet.
(11 February 1944 to an individual believer)
We cannot be sure of the authenticity of any of the phrases in the Old or the New Testament. What we can be sure of is when such references or words are cited or quoted in either the Quran or the Bahá'í writings.
(4 July 1947 to an individual believer)
Except for what has been explained by Bahá'u'lláh and 'Abdu'l-Bahá, we have no way of knowing what various symbolic allusions in the Bible mean.
(31 January 1955 to an individual believer)
From letters written on behalf of the Universal House of Justice:
The interpretation of biblical prophecies has long been the subject of controversy and speculation among religious scholars. As Bahá'ís, we know that we must turn to the Writings of Bahá'u'lláh, 'Abdu'l-Bahá and Shoghi Effendi for authoritative guidance in these matters. When a subject has not been mentioned or explained in the Sacred Writings, we are free to consult other books and to consider the opinions of scholars if we wish to do so.
In studying the Bible Bahá'ís must bear two principles in mind. The first is that many passages in Sacred Scriptures are intended to be taken metaphorically, not literally, and some of the paradoxes and apparent contradictions which appear are intended to indicate this. The second is the fact that the text of the early Scriptures, such as the Bible, is not wholly authentic.
(28 May 1984 to an individual believer)
...The Bahá'ís believe that God's Revelation is under His care and protection and that the essence, or essential elements, of what His Manifestations intended to convey has been recorded and preserved in Their Holy Books. However, as the sayings of the ancient Prophets were written down some time later, we cannot categorically state, as we do in the case of the Writings of Bahá'u'lláh, that the words and phrases attributed to Them are Their exact words.
(9 August 1984 to an individual believer)
The Bible: Extracts on the Old and New Testaments