And mankind was not but one community [united in religion], but [then] (10:19)

Almuntakhab Fi Tafsir Alquran Alkarim

People were but one single nation, then they differed among themselves in opinion and each group cherished a certain view and belief and were altogether set at variance. Had it not been for Allahs word proclaimed beforehand to delay the decision on matters of doubt and dispute among them and to put punishment in respite, their Hereafter would have been planted in the now

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(10:19) Once all men were but a single community; then they disagreed (and formulated different beliefs and rites). *25 Had it not been that your Lord had already so ordained, a decisive judgement would have been made regarding their disagreements. *26

And mankind was not but one community [united in religion], but [then] meaning

*25). For further elaboration see Towards Understanding the Qur'an, vol. I, al-Baqarah 2, n. 230, pp. 165-6, and ibid., vol. II, al-An 'am 6, n. 24, pp. 228-9 -Ed.
*26). The fact is that God has wilfully kept reality beyond the ken of man's sense-perception. This was done so as to test whether man, by making proper use of his reason, conscience and intuition, is able to find the right way. As a logical corollary of this, God allows those who decide to follow any other way rather than the right one to do so. Had this not been the case, the reality could have been instantly unmasked and that would have put an end to all disagreements relating to the issue.
This statement has been made in order to remove a major misconception. Many people feel disconcerted today, as they felt disconcerted at the time when the Qur'an was revealed, by the fact that there are found different religions in the world and that the followers of each of these are convinced that their religion alone is the true one. People have been puzzled as to how to determine which religion is the right one and which is not.
In this regard, the Qur'anic view which is being articulated here is that multiplicity of religions does not constitute the original state of things, that it is a relatively later development in human history. Initially, all mankind professed a single religion, and that religion was the true one. Later on, people began to disagree regarding the truth and this gave rise to different religions and creeds. Now it is not likely that God would dramatically appear before them and reveal the reality to them directly during the present phase of their worldly existence. For, the very purpose of the present life is to test people. And this test consists of finding out who discovers the reality - even though it is beyond the reach of the senses - with the help of his reason and common sense.
 

And mankind was not but one community [united in religion], but [then] meaning in Urdu

ابتداءً سارے انسان ایک ہی امّت تھے، بعد میں انہوں نے مختلف عقیدے اور مسلک بنا لیے، اور اگر تیرے رب کی طرف سے پہلے ہی ایک بات طے نہ کر لی گئی ہوتی تو جس چیز میں وہ باہم اختلاف کر رہے ہیں اس کا فیصلہ کر دیا جاتا