Christianity definitely created what we understand as liberalism today, which is really radical egalitarianism (not always very liberal in practice).

However, it was a Jewish convert named Saul who laid the foundation:

There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus.

Our contemporary liberalism started with a repudiation of Classical Judaism, which as a highly stratified, racially exclusive religion could not be liberal by Western standards. Nor can modern Judaism or Islam, both of which reject Paul’s proclamation that we are all equal under God. But it took over a thousand years of institutional Christianity before the idea mutated into its current form.

My guess is that as long as the church had a great deal of authority over people’s lives, it was a useful concept for promoting social harmony and a sense of mutual obligation among the different classes (e.g. aristocrats were supposed to acknowledge that God did not favor them over social inferiors), as well as a check on secular power vis a vis the church. However, when the church’s power began to recede and break up as it lost its monopoly over knowledge, this central tenet of Christianity took on a life of its own – it came unmoored from the rock of faith so to speak – and became a tool in the hands of secular factions challenging the prevailing authority.

Jews could not develop a Western-style liberal society as long as they lived under rabbinic authority, so it was only when egalitarian Westerners freed them from that authority (a sort of ecclesiastic law for Jews) and granted them citizenship that they were able to do so.

And no, the liberalism wasn’t the Protestants’ fault. Napoleon was more influential than anyone else in spreading the new faith. I think the peculiar Yankee and English form of radical egalitarianism was a competition with the French; an attempt to show that they were just as righteous as the French – more so even – and could prove it. As an example of the ferocity of this competition, in the beginning of the American Republic the Francophile and Anglophile factions waged what amounted to a low-intensity war. So many killed each other in duels that the officer corps was depleted.

Today all egalitarian political systems, including our own as well as Communist China’s, are based on this originally Christian idea of human equality. We have been in a crusade that has been on the march for over 200 years. It is very much a religious movement, and so all-encompassing that it rarely even occurs to people to question their faith in the beliefs it is built upon. Only in the last few decades, and ever so carefully, have a few begun to do so openly.