Allah could have made them one community

وَأَنزَلْنَآ إِلَيْكَ ٱلْكِتَـٰبَ بِٱلْحَقِّ مُصَدِّقًا لِّمَا بَيْنَ يَدَيْهِ مِنَ ٱلْكِتَـٰبِ وَمُهَيْمِنًا عَلَيْهِ ۖ فَٱحْكُم بَيْنَهُم بِمَآ أَنزَلَ ٱللَّهُ ۖ وَلَا تَتَّبِعْ أَهْوَآءَهُمْ عَمَّا جَآءَكَ مِنَ ٱلْحَقِّ ۚ لِكُلٍّ جَعَلْنَا مِنكُمْ شِرْعَةً وَمِنْهَاجًا ۚ وَلَوْ شَآءَ ٱللَّهُ لَجَعَلَكُمْ أُمَّةً وَٰحِدَةً وَلَـٰكِن لِّيَبْلُوَكُمْ فِى مَآ ءَاتَىٰكُمْ ۖ فَٱسْتَبِقُوا۟ ٱلْخَيْرَٰتِ ۚ إِلَى ٱللَّهِ مَرْجِعُكُمْ جَمِيعًا فَيُنَبِّئُكُم بِمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ

We sent to you [Muhammad] the Scripture with the truth, confirming the Scriptures that came before it, and with final authority over them: so judge between them according to what God has sent down. Do not follow their whims, which deviate from the truth that has come to you. We have assigned a law and a path to each of you. If God had so willed, He would have made you one community, but He wanted to test you through that which He has given you, so race to do good: you will all return to God and He will make clear to you the matters you differed about.

وَلَوْ شَآءَ رَبُّكَ لَجَعَلَ ٱلنَّاسَ أُمَّةً وَٰحِدَةً ۖ وَلَا يَزَالُونَ مُخْتَلِفِينَ

If your Lord had pleased, He would have made all people a single community, but they continue to have their differences-

وَلَوْ شَآءَ ٱللَّهُ لَجَعَلَكُمْ أُمَّةً وَٰحِدَةً وَلَـٰكِن يُضِلُّ مَن يَشَآءُ وَيَهْدِى مَن يَشَآءُ ۚ وَلَتُسْـَٔلُنَّ عَمَّا كُنتُمْ تَعْمَلُونَ

If God so willed, He would have made you all one people, but He leaves to stray whoever He will and guides whoever He will. You will be questioned about your deeds.

وَلَوْ شَآءَ ٱللَّهُ لَجَعَلَهُمْ أُمَّةً وَٰحِدَةً وَلَـٰكِن يُدْخِلُ مَن يَشَآءُ فِى رَحْمَتِهِۦ ۚ وَٱلظَّـٰلِمُونَ مَا لَهُم مِّن وَلِىٍّ وَلَا نَصِيرٍ

If God had so pleased, He could have made them a single community, but He admits to His mercy whoever He will; the evildoers will have no one to protect or help them.