Christians found virtuous Muslim deeds impressive precisely because they deemed them vain

Evidence

The virtuous actions of Muslims were certainly thought of as vain; that was what made them so surprising, so impressive to Christians, for they could not avail to salvation. — Norman Daniel, Islam and the West, 1278

Categorization

• DEEPTHINKING: A genuinely counterintuitive inversion: the Christian judgment that all Muslim virtue was ‘vain’ is presented as the very reason such virtue could be admired — the futility of the act made its performance more astonishing, not less. A compact paradox about how disparagement can coexist with awe.