How did the Islamic creed start?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

The Islamic creed is the creed of all the Prophets and Messengers, from Aadam (Adam) until the last Messenger Muhammad, sallallaahu ‘alayhi wa sallam. Allaah The Almighty Says (what means): {And whoever desires other than Islam as religion— never will it be accepted from him, and he, in the Hereafter, will be among the losers.} [Quran 3:85] 

The Quran tells us that Nooh (Noah) said (what means): {"And I have been commanded to be of the Muslims."} [Quran 10:72]

Ibraaheem (Abraham) said (what means): {"I have submitted [in Islam] to the Lord of the worlds."} [Quran 2:131]

Yoosuf (Joseph) said (what means): {"Cause me to die a Muslim and join me with the righteous."} [Quran 12:101]

The Prophet, sallallaahu ‘alayhi wa sallam, said that all the Prophets are Muslims, yet Allaah The Almighty sent them with different laws. [Al-Bukhari and Muslim]

However, some followers of the Prophets did not adhere to what Allaah The Almighty sent them with, and they committed Shirk (polytheism). Therefore, Allaah The Almighty sent one Prophet after another to correct their deviations. The last Prophet was Muhammad, sallallaahu ‘alayhi wa sallam.

Allaah The Almighty protected Islam against distortion.  Allaah The Almighty Says (what means): {Indeed, it is We who sent down the Thikr and indeed, We will be its guardian.} [Quran 15:9]

"The Thikr" refers to the Quran and Sunnah (Prophetic tradition), which is the basis of Islam.

The nation of the Prophet, sallallaahu ‘alayhi wa sallam, as a whole does not agree upon any act of deviation; however some Muslims, not all, may commit deviations, but there are Muslims who will remain steadfast in doing righteous deeds until the Day of Resurrection, as the Prophet, sallallaahu ‘alayhi wa sallam said. [Al-Bukhari and Muslim]

Your question can be interpreted in two ways: first, what are the first established facts of the Islamic creed? There was no beginning to such facts because they are eternal facts related to the Essence of Allaah The Almighty, His Names and His Attributes and their implications and effects. Second, when did humans come to know these facts? The answer is mentioned in the introduction. Aadam as well as all the Prophets and Messengers, were Muslims. Indeed the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said that Aadam was a Prophet whom Allaah The Almighty talked with. [Ahmad]

Allaah Knows best.