Mankind was but one nation, but differed (later). Had it not been for a word that went forth before from thy Lord, their differences would have been settled between them. Yunus 10:19 ⧉
Tafsir:
God initially grounded human existence in the religion of nature. At that time, the religion of all humanity was one and the same. But, people had their differences and devised different forms of religions. The reason for this is the misuse of the freedom given to the people for the purpose of testing them. If God were to reveal Himself, human arrogance would come to an end on observing His powers, and suddenly unity would replace differences, because extremity of fear demolishes a multiplicity of opinions. But God will not intervene in this state of affairs before Judgement Day. God had created the present world for the sake of putting human beings to the test, and to maintain the atmosphere of testing, it is necessary to keep the reality concealed, so that people should have the opportunity to use their wisdom in both right and wrong directions.