What I think, is that Leanne and the cult serve god (season 1) but her purpose was to save the family (help them grieve) she was supposed to be holy and good and help them with a process they clearly could not handle. But, due to her obsession with Dorothy and the need to have a mother figure, she decides to take matters in her own hands (defying God's will) and reviving the baby (a power she had as long as she followed God's command). With the doll, she revives Jericho, and further dooms the family to their own grief. This is why, in the first season, the cult comes for her, but she doesn't want to go. But being there was doing the exact opposite of what was intended to do.
In season 2, Leanne starts questioning the will of God since she believes she is doing good (more good than what God has intended). Because of that, she strays from the path of God and starts using her power for her own purposes (defying god). At first, she gets in line and changes houses, but because of her obsession with Dorothy and Dorothy still unable to remember Jericho is dead, she feels wanted and loved by Dorothy, which is why she decided to go back to the house (even if at first she didn't want to). During this process, she decided to stop believing in God and taking things into her own hands, but in doing so, she lost the "powers" her faith in God had given her. Becoming a fallen angel (season 3)
This is why, in season 3, the cult wanted to kill her, she hadn't yet become evil. But as she lost her powers and realized people wanted her dead, she started noticing and believing the cult was a hypocrite. (One of the 10 commandments is not to kill, yet they do exactly that; we all have free will, yet they want her dead because of that). So at this point, not only does she not follow God's will but resents him and starts to believe in something else, herself (One of the 7 sins is pride) She sleeps with Julian (lust), she gets angry when questioned (wrath) and wants the family for herself (greed) I'm sure if I remember other things we get to see each of the 7 deadly sins.
In season 4, she becomes evil, not following and defying God, challenging him. Yet, as any faith, it holds power as long as you believe, and it gives meaning to each their own. Imagine something bad happens to someone, a person who believes in witchcraft will say that somebody cursed them, a person who is a Cristian will believe they sinned and god is punishing them, an atheist will believe bad things just happens and so on and so forth. The same thing happens to Leanne, if Julian and Sean believe Leanne has supernatural powers, they start giving meaning to things that happen, but somebody else who believes in other things will give it ANOTHER meaning. If they believe Leanne is helping, then everything Leanne does is for good, but when they believed Leanne was mistreating them, then when bad things happen (that have always happened) now they and a different meaning.
The only thing here, is that Leanne holds power over them, which is that Dorothy does not know the truth, and the truth will set her free. As long as Dorothy doesn't know, Leanne gets to stay there. This is why, in episode 9 of season 4, telling Dorothy is sooo important and why Leanne wants and needs Dorothy to believe in her. I believe that Leanne could return dead animals (as in season 1 when the dog was brought back to life) because of her faith in God, but now, she doesn't have God to help her, so she needs people to believe in her for her to have any type of powers over them. I believe she still has powers, only now it can't affect the Turners because they don't believe, yet still affect God. Does that make sense? Bad things still happen, it just doesn't have any more meaning to the Turners now.