In The Poisonwood Bible does Leah ever think, feel, and act like herself or does she simply replace one authority figure with his cause for another?
That is an interesting question, one that really makes you think about Leah's character. At the beginning of the novel, Leah adores her father and is a strong advocate of his cause. However, once they move to the Congo, it doesn't take her long to realize how foolish and out of touch he really is; she turns from him, and chooses another path. That path happens to be activism for the African plight. So, she was a rather devout follower of her father's cause, until she turned into being a devout follower of Africa's causes with her husband, Anatole.
I think that you could argue the question either way. On the one hand, Leah makes Africa and suffering there her entire identity, choosing to raise and rear her children there, even though she has the opportunity to do so in America where the situation is much better. When she comes to America, she feels a bit lost, like a ship without an anchor. Africa's causes root and ground her; she is a woman who needs a purpose and a fight. Africa provides one for her. So in that sense, she does lose her identity--it becomes swallowed in causes larger than herself. However, on the other hand, who is to say that there isn't a separate identity in that? Who is to say that Leah doesn't feel her true character, her strength, her resilience and her endurance, through her struggles with a cause? Some people identify with that; Leah was at her most powerful when she was working hard for her passionate beliefs. That is an identity--more than most find in their lives. Most people float through life without any direction or sense, and at least Leah had some. She knew from a very early age that she was a fighter, a passionate one at that, and I don't think she would've ever been happy or truly herself without a cause to fight. That was a huge part of who she was.
I know that I didn't conclusively answer the question either way, but I think this question lends itself well to healthy debate on either side. I hope I've given you enough to get your wheels turning; good luck!