From one review of a related book:
The discovery of the individual was one of the most important cultural developments in the years between 1050 and 1200. It was not confined to any one group of thinkers. Its central features may be found in different circles: a concern with self-discovery; an interest in the relations between people, and in the role of the individual within society; an assessment of people by their inner intentions rather than by their external acts. These concerns were, moreover, conscious and deliberate. ‘Know yourself’ was one of the most frequently quoted injunctions. The phenomenon which we have been studying was found in some measure in every part of urbane and intelligent society.That review ties it into outbreeding v. nepotism, guilt v. shame, and rule of law v. feuding.
Boston Review is unpersuaded:
during the same Middle Ages when Christianity was supposedly becoming modern liberalism below the surface, its adherents dedicated themselves to crusading violence abroad and principled intolerance at home. When Siedentop alludes to the Crusades it is to remark on how they unified Europe and encouraged knights to put their petty feudalism aside in order to agree that Christians should never kill fellow Christians — as if the main problem were not how medieval Christians learned to tolerate other sorts of people or understand the rest of humanity to be on par with themselves. “Strikingly, in its first centuries Christianity spread by persuasion, not by force of arms — a contrast to the early spread of Islam,”I guess the reviewer wants to blame Christianity for fighting Islam. I don't know why -- Islam certainly was not going to invent individualism or modern liberalism, and Christianity was not going to either unless it was willing to repel Islamic invaders.
I have not read the book, and there are other theories for modernity. Civilization more than two millennia ago was concentrated in the middle east, from Egypt to Babylonia to Persia to India, and in China. Then came the Greeks, Christianity, and the Romans. And Europe advanced very rapidly in the last 500 years or so.
There were also a bunch of changes in Europe 1000 years ago or so, such as banning cousin marriages and an economy based on individual families. These books probably explain it.