How did diversity cause nationalism to
rise in these different empires?

Diversity is a sociological concept that implies an ethnic, cultural and social mixture in a given society. In the past, before the rise of modern migratory currents, nations were characterized by their homogeneity, that is, by the fact that their inhabitants belonged in their vast majority to the same ethnocultural group, thus sharing physical, religious, social and cultural characteristics.

But as the world became interconnected, the population exchange between nations began. This was not well received in all of them, but in some cases immigration was seen as a threat to the aforementioned national values. For this reason, nationalist movements began to develop that began to promote the protection of these values, as well as the rejection of those values brought by immigrants.

the covenant that god made with abraham was a unilateral (one-sided) promise from god to abraham.   apart from anything abraham did, god was promising that he was going to bring about a great nation from abraham's offspring, and that the messiah (meaning "the anointed one") would come from abraham's descendants, as one through whom the whole world would receive favor from god.   as god promised abraham, "through your offspring all nations on earth will be blessed" (genesis 22: 18).

the "new covenant" was announced by jesus of nazareth, who had asserted that he was the "son of david" (and descendant of abraham) that fulfilled those promises to abraham.   when jesus introduced the sacred meal that we now refer to as the lord's supper, as told his disciples in reference to the wine they drank, "this cup is the new covenant in my blood, which is poured out for you" (luke 22: 20).   this "new covenant" through jesus was also a unilateral (one-sided) covenant.   he gave himself in sacrifice for all people, without conditions attached.   as his friend and apostle john later wrote, "he is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world" (1 john 2: 2).

so the covenant with abraham and the "new covenant" ushered in by jesus are, in essence, connected as one covenant -- because jesus was saying he was the one promised to abraham that fulfilled that covenant and brought blessing to the world.