Unity by free will & choice
Unity among diverse groups of people differing in belief cannot be forced; it has to come by free will and choice of the people, as enshrined:
‘’Mankind was a single community; then they differed. Were it not from a prior decree from your Lord, the matters over which they had disputed would have been settled’’ (10: 19)
The significance of the Holy Verse lies in the fact that in spite of an enforced unity being very much within the prowess of the Lord of the universe; it was left to be devised by free will and choice of diverse groups of people differing in belief. It is clearly ordained that the matters over which people differed would have been settled, were it not from a prior decree of Lord.
The prior decree, as it stands, rules out an enforced union. Instead, the decree opts for diverse groups of people uniting by free will and choice. The decree thus sought to unite people by prophetic missions. The prophets carried the divine message inviting people to shape their lives, as per the divine commands.
The ultimate objective of prophetic missions, including the mission of Prophet of Islam-Hazrat Mohammad (SaW) was to eliminate the differences among human beings and make them one Ummah, as was the order at the very initiation of human race, when mankind constituted a single community. The differences cropped up and they split over matters of belief and faith.
The divine intervention did not come about in enforcing the union of people. Instead, it came about in the form of healing process. The process worked on easing out whatever wrong had crept in over a period of time and substituting it gradually with whatever was deemed right in the divine order.
This was thus the prime objective of prophetic missions in different epochs, entailing union of people by working on their sensitivities, getting free will into play to make the right choice, and tread the evened path.