My reply:
Thanks Silverbackman for your post! and you are most welcome here.
Good question!
Actually there are several Manifestations of God mentioned in the Baha'i Writings but we are told there were more than that. It's just that many are unknown to us...
In the Baha'i Faith, a Manifestation of God is a Special Being Who we believe perfectly reflects the Light of God and from Whom a new dispensation follows and that there are lesser Prophets who are in their turn dependent on Them....
We believe there were Manifestations of God and prophets before in remote ages of antiquity and that there will be more stretching into the future.
See the following:
http://bahai-library.com/encyclopedia/agescycl.html
While Guru Nanak was not specifically mentioned in the core of our Writings, see the following article for a Baha'i view of Guru Nanak:
http://bahai-library.com/books/miracles/nanak.html
Baha'is maintain good relationships with Sikhs and work with them in Inter-Faith activities.
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Also for your further information, Lao Tse and Confucius are not considered Prophets by us. See the following:
"Regarding Lao-Tse: the Bahá'ís do not consider him a prophet, or even a secondary prophet or messenger, unlike Buddha or Zoroaster, both of Whom were divinely appointed and fully independent Manifestations of God....
From a letter written on behalf of Shoghi Effendi - 10 November 1939
"Confucius was not a Prophet. It is quite correct to say he is the founder of a moral system and a great reformer."
From a letter written on behalf of Shoghi Effendi - 26 December 1941 to the National Spiritual Assembly of Australia and New Zealand