Pierre Manent begs more than he explains:
What to do about our diminished collective capacity is the great political question of Europe. Whether in relation to European unification or to Islam, it is clear that we have nothing pertinent to say if we refrain from making claims about European identity. One way to outline essential characteristics of European political and spiritual life is to contrast them with certain fundamental features of Muslim life.
Running the risk of a somewhat rough stylization, we might say something like the following: Islam throughout its history has largely preserved the form, the impulse, and the consciousness of empire (traits that are found with renewed vigor today), while Western Christianity, though born in an imperial form, and very much subject to great missionary and conquering movements, found its relative stability in a very different arrangement. Islam was never able to abandon the imperial form that Christianity could never assume in a lasting way. Christianity instead found its form in the nation, or in the plurality of nations once called “Christendom,” then “Europe.”
Anyone who reads the first pope Peter, finds little sense of Christian empire:
But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light. Once you were not a people, but now you are God’s people; once you had not received mercy, but now you have received mercy.
Beloved, I urge you as sojourners and exiles to abstain from the passions of the flesh, which wage war against your soul. (1 Peter 2:9-11 ESV)
The early church thought of itself more in exile than as part of the establishment. Then came Constantine. Then came empire. Then came establishment. Only with the novos ordo seclorum did Europeans begin to question the fit between Christianity and empire.