Yunus 10:19

The whole of mankind was but a single community, then they differed[21]. If it had not been for a prior decree from your Lord, a decisive judgment would have been passed between them concerning their differences[22].

[21] Ibn ‘Abbās (رضى الله عنهما) reported: “The time between Adam and Noah was ten centuries. Throughout these, people were on the right path of Allah, but then they started disputing [over matters of Faith, i.e., idol worshiping]. So, Allah sent prophets bearing glad tiding and warning”. [Al-Hākim, Al-Mustadrak: 2:546, cf. Al-Tabarī]
[22] i.e., not to punish anyone before evidence has come to him, or not to judge between them in this life.

And mankind was not but one community [united in religion], but [then] they differed. And if not for a word[517] that preceded from your Lord, it would have been judged between them [immediately] concerning that over which they differ.

[517] Allāh's decree to allow time on earth for His creation or not to punish anyone before evidence has come to him.

19. Mankind were but one community (i.e. on one religion - Islâmic Monotheism), then they differed (later); and had not it been for a Word that went forth before from your Lord, it would have been settled between them regarding what they differed[5].

[5] (V.10:19) Narrated Abu Hurairah رضي الله عنه: The Prophet صلى الله عليه وسلم said, "Every child is born on Al-Fitrah [true faith of Islâmic Monotheism (i.e. to worship none but Allâh Alone)], but his parents convert him to Judaism or Christianity or Magianism, as an animal gives birth to a perfect baby animal. Do you find it mutilated?" (Sahih Al-Bukhari, Vol.2, Hadith No.467). [See also the Qur’ân, Verse 30:30]

Mankind were but one community; then they differed; and had it not been for a word that had already gone forth from thy Lord it had been judged between them in respect of that wherein they differ.