And if the enemy had entered from all sides (of the city), and they had been exhorted to AlFitnah (i.e. to renegade from Islam to polytheism) they would surely have committed it and would have hesitated thereupon but little.
Meaning:-
Exhorted: strongly encourage or urge (someone) to do something.
AlFitnah: ( Al-Fitnah is worse than killing.) means, trying to force the Muslims to revert from their religion and re-embrace Kufr after they had believed, is worse with Allah than killing.