Article one traces the doctrine's roots into Islamic tawhid and Shaykhi theology. Its introduction to a Western audience has its own separate and more precisely dated history, and it did not happen through Baha'i missionaries at all.
The 1893 World's Parliament of Religions in Chicago, convened alongside the World's Columbian Exposition and widely regarded as the founding event of organised interfaith dialogue in the West, is where the Baha'i teaching of the oneness of God first reached significant American public attention, when the Reverend Henry Harris Jessup, a Presbyterian missionary long resident in Beirut, read a paper describing the Babi movement and quoting a message attributed to Baha'u'llah calling for the unity of all religions and peoples under the one God worshipped, in different names, by them all. Jessup had not intended to promote the teaching he described, but newspaper coverage of the Parliament carried the message to American readers who then sought out further contact, and small Baha'i study circles began forming in the United States within a few years.
The episode is a useful check on how "oneness of God" travels compared with how it is argued for internally. Article one's account rests on Baha'u'llah's own writings and their Islamic and Sufi background; the doctrine's actual entry into Western religious consciousness came instead through a hostile external observer's report at a comparative-religion event whose own founding premise, that the world's faiths could be surveyed together on a shared public platform, was itself a working instance of the same impulse toward religious unity the doctrine describes.