Islam recognized Christendom as its only genuine rival civilization early on.

Evidence

Like every other civilization known to human history, the Muslim world in its heyday saw itself as the center of truth and enlightenment, surrounded by infidel barbarians whom it would in due course enlighten and civilize. But be­ tween the different groups of barbarians there was a crucial difference. The bar­ barians to the east and the south were polytheists and idolaters, offering no serious threat and no competition at all to Islam. In the north and west, in contrast, Muslims from an early date recognized a genuine rival—a competing world religion, a distinctive civilization inspired by that religion, and an empire that, though much smaller than theirs, was no less ambitious in its claims and aspirations. This was the entity known to itself and others as Christendom, a term that was long almost identical with Europe. — Bernard Lewis, From Babel to Dragomans, 1543

Categorization

• XREF: Provides the symmetry premise for the whole essay: Islam regarded Christendom the way Christendom regarded Islam — supports later claims about mutual ignorance and mirroring. Fits the reader’s Islam-West representation cluster as background on why the two civilizations were locked in attention.
• corroborated: 2 independent passages support this claim — Bernard Lewis, From Babel to Dragomans: Interpreting…, 1543