Baptism Scene

In religion, a backslider is someone who has turned their backs on their religious beliefs and acted in a way that is not compatible with their religious beliefs.

In the context of the "There Will Be Blood" baptism scene, when Eli says "You brought your son for me to bless, and you brought your new partner. And you brought bad habits as a backslider," he is using the term "backslider" to refer to a person who has lapsed in their religious beliefs or practices.

In Christian theology, a backslider is someone who has fallen away from their faith or has returned to sinful ways after a period of spiritual progress. The term is often used to describe someone who was once a committed follower of Christ but has since turned away from their faith.

Eli is using the term "backslider" to criticize Daniel Plainview for not being a devout Christian and for engaging in unscrupulous business practices.

He implies that Daniel is a hypocrite for having his son baptized because his actions are not in line with the teachings of the Christian faith.

In general, the use of the term "apostate" in this context is meant to condemn Daniel Plainview's character and actions.