Mary Baker Eddy was born Mary Morse Baker in Bow, New Hampshire,the youngest of six children of Abigail and Mark Baker. Although raised a Congregationalist, she came to reject teachings such as predestination and original sin. She suffered chronic illness and developed a strong interest in biblical accounts of early Christian healing. At the age of eight, Eddy began to hear voices calling her name; she would go to her mother only to learn that her mother had not called her. In her autobiography, Eddy relates one of these later experiences:

"One day, when my cousin, Mehitable Huntoon, was visiting us, and I sat in a little chair by her side, in the same room with grandmother,-the call again came, so loud that Mehitable heard it, though I had ceased to notice it. Greatly surprised, my cousin turned to me and said, 'Your mother is calling you!' Finally, after speaking with her mother, the child Mary responded to the voice with the phrase from Samuel 'Speak, Lord; for Thy servant heareth.' When the call came again I did answer, in the words of Samuel, but never again to the material senses was that mysterious call repeated."
Eddy frequently expressed confidence in God's love, which placed her at odds with her father's theological outlook, leading to a religious crisis when she was twelve and first eligible to join the Congregational church. Mark Baker held to a hard and bitter doctrine of predestination, believing that a horrible decree of endless punishment awaited sinners on a final judgment day. Amid their clash of views Mary developed a life-threatening fever, which at last prompted her father, in his love for her, to set aside his stern beliefs. She was healed of the fever after prayer, as she wrote: "My mother, as she bathed my burning temples, bade me lean on God's love, which would give me rest if I went to Him in prayer, as I was wont to do, seeking His guidance. I prayed; and a soft glow of ineffable joy came over me. The fever was gone and I rose and dressed myself in a normal condition of health. Mother saw this and was glad. The physician marveled; and the "horrible decree" of Predestination - as John Calvin rightly called his own tenet - forever lost its power over me."