The Quran

وَما كانَ النّاسُ إِلّا أُمَّةً واحِدَةً فَاختَلَفوا ۚ وَلَولا كَلِمَةٌ سَبَقَت مِن رَبِّكَ لَقُضِيَ بَينَهُم فيما فيهِ يَختَلِفونَ

Muhammad Asad

AND [know that] all mankind were once but one single community, and only later did they begin to hold divergent views.¹ And had it not been for a decree - that had already gone forth from thy Sustainer, all their differences would indeed have been settled [from the outset].²

Lit., ("and then they disagreed [among themselves]". For an explanation of the term "one single community" (ummah wahidah), see Sarah 2, note 197. In the present context, this expression alludes not merely to mankind's one-time homogeneity, but also - by implication - to the fact, repeatedly stressed in the Qur'an (e.g., in 7:172), that the ability to realize God's existence, oneness and omnipotence is innate in man, and that all deviation from this basic perception is a consequence of the confusion brought about by man's progressive estrangement from his inborn instincts.
Lit., "it would indeed have been decided between them regarding all that they were differing in": i.e., had it not been for God's decree - which is the meaning, in this context, of the term kalimah (lit., "word") - that men should differ in their intellectual approach to the problems touched upon by divine revelation, "they would not have contended with one another after having received all evidence of the truth", but would all have held from the very outset, and would continue to hold, the same views (cf. 2:253 and the corresponding note 245). Since, however, such a uniformity would have precluded men's intellectual, moral and social development, God has left it to their reason, aided by prophetic guidance, gradually to find their way to the truth. (See also surah 2, note 198.) The above parenthetic passage must be read in conjunction with 2:213.