Eliza's faith has made her an obedient slave to the good Mrs. Shelby. As Eliza puts it to George, who has decided to run away:
"Well," said Eliza, mournfully, "I always thought that I must obey my master and mistress, or I couldn't be a Christian."
Eliza's Christianity also helps save her from the bitterness that eats away at George for being the slave of a cruel master. Eliza has been fortunate in her situation, but even after Mr. Shelby does a cruel thing in selling her four-year-old son Harry to a slave trader, Eliza is able to avoid deep bitterness through faith.
Her Christianity gives her the "grit," as Mr. Symmes later calls it, to flee with her child. She feels God is with her as she runs:
... she felt the weight of her boy as if it had been a feather, and every flutter of fear seemed to increase the supernatural power that bore her on, while from her pale lips burst forth, in frequent ejaculations, the prayer to a Friend above -- "Lord, help! Lord, save me!"
When Harry asks her is she is sure they will escape, she feels a supernatural conviction that they will:
"Yes, sure!" said the mother, in a voice that startled herself; for it seemed to her to come from a spirit within ...
While Stowe's use of Christianity in the novel has been condemned for helping slaves accept their lot, Stowe understands it as a source of strength and empowerment for them.
I think that the most profound effect that Christianity had on Eliza was to help to develop her sense of self- sacrifice. Eliza sacrifices everything in order to save her family. This is reminiscent of the Christian ideal of sacrifice in which individuals struggle in the name of a higher good and higher force. For Eliza, this is family. Her flight across the Ohio River in the cold embodies this. Eliza suffers greatly, but does not lose her overall faith and commitment in the sacrifices she needs to make for her family. In saving Harry, Eliza assumes a Christian notion of sacrifice. While her reaction is instinctual as a mother for a child, one sees the Christian notion of sacrifice of oneself for something higher in such an action. Eliza does not succumb to the fear and anger that the condition of slavery would understandably arouse in a slave. It is through her own Christian conception as part of her own consciousness where she is able to transcend the wrongs done to her and act in accordance to a good for someone else and something else larger than herself. In this, one can see the effect that the Christian religion had on Eliza.