Final answer:
Unferth challenges Beowulf's credibility due to his perceived arrogance.
Step-by-step explanation:
The quality that made Unferth challenge Beowulf's credibility, as subtly pointed out by the author, is arrogance. In the epic poem Beowulf, Unferth questions Beowulf's claims of greatness and heroism because he perceives Beowulf's confidence as arrogant boasting. Unferth's challenge highlights a theme of pride and its potential negative effects on one's credibility.