Mary Rowlandson quotes 2 Corinthians 12:9 ("And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness") as she describes her sister's death to highlight her reliance on God's grace and strength during her difficult experiences. By quoting this verse, Rowlandson emphasizes her belief in God's sovereignty and how she finds solace and comfort in her faith. It serves as a reminder of her spiritual resilience and a source of strength to endure the hardships she faced. Additionally, it could also be seen as a way for Rowlandson to cope with the loss of her sister, finding consolation in the notion that her sister's weakness was made perfect in God's strength and grace.
