According to the commentaries, the verse addresses the tribes of Aws and Khazraj in Medina, who had fought one another for years; it goes on to say, “you were enemies, and He reconciled your hearts, and you became brothers.”
The image of the rope is fitting: one who holds on alone may fall; those who hold on together carry one another too.

The section opened with a call to know one another; it closes with the command not to be divided.
Sa'di's limbs, Donne's continent, the saying Tutu passed on are fibers of the same rope.

To be one is not to let go of what you hold on to.