Final answer:
Beowulf's mail shirt was most responsible for saving his life in the battle against Grendel's mother. Divine intervention did not play a role in this particular instance.
Step-by-step explanation:
In the battle against Grendel's mother, Beowulf's mail shirt played a significant role in saving his life. Grendel's mother was unable to penetrate the mail shirt with her weapon. This is evident when the text mentions that Beowulf was able to remove the carcass of Grendel's mother without any harm to himself.
While Beowulf's sword, Hrunting, initially failed to defeat Grendel's mother, it was ultimately another weapon, a magical sword he found in her lair, that helped him land the winning blow. However, his mail shirt was crucial in ensuring his survival during the fight.
Divine intervention, though a common theme in epic literature, does not play a role in saving Beowulf's life in the battle against Grendel's mother.