[23:52] And verily this Brotherhood of yours is a single Brotherhood, and I am your Lord and Cherisher: therefore fear Me (and no other).
[23:53] But people have cut off their affair (of unity), between them, into sects: each party rejoices in that which is with itself.
Bani Israel cut themselves into groups despite all their respective messengers bringing them a single message.
[3:103] And hold fast, all together, by the rope which Allah (stretches out for you), and be not divided among yourselves; and remember with gratitude Allah’s favour on you; for ye were enemies and He joined your hearts in love, so that by His Grace, ye became brethren;..
The first principles of existence or coming together of Muslim Umma is highlted by Quran as love and brotherhood.
[5:14] From those, too, who call themselves Christians, We did take a covenant, but they forgot a good part of the message that was sent them: so we estranged them, with enmity and hatred between the one and the other, to the day of judgment…
Forgetting a part of covenant with God results in animosity and hatred between the Christians.