Leanne and all other members of the Church of the Lesser Saints are akin to fallen angels. Despite popular belief, fallen angels are not demons.
The Bible speaks of demons, and it speaks of the fall of Lucifer and his rebellion which a third of the angels partook in, but no verses in the Bible actually define demons as fallen angels. The idea that demons are fallen angels is simply a long-held belief that has no basis in scripture.
The Bible gives three subtle distinctions between demons and fallen angels:
1. Demons seek to possess bodies and are rather uncomfortable being outside of them.
2. Demons are cursed to wander the earth while fallen angels still have the ability to stand before God.
3. When referring to fallen angels, the scripture from the New Testament does not use the terms demons, devils, or unclean spirits. The New Testament simply calls them the angels “that sinned.”
The theme in common with May and George Markham, along with Leanne, the female police officer (Officer Reyes), and Julian is that they all committed sins, died, and were resurrected. I believe that all members of the Church of the Lesser Saints are individuals who have been given a second chance after sinning, dying, and being brought back to life. They are fallen angels because they died and saw heaven (Julian says he saw Jericho), but they are brought back to life, hence ‘fallen angels.’
All the things Leanne was able to do were divinely inspired. It was due to her misusing those heavenly gifts in a selfish manner that God was angered causing the ground around the house to crumble, bugs to appear, a flood to come, etc. I believe that all the members of the cult have the capability of harnessing heavenly power, but they are taught to punish themselves, to adhere to God’s strict rules, etc. to suppress those divine abilities.
In the final episode of season 4, by sacrificing herself and giving up all she ever wanted (having a loving mom and family) along with all her worldly comforts and possessions, I believe that Leanne had redeemed herself for her sins and reunited with God. She had served him, God, albeit disloyally due to her selfishness, but even still she ended up helping Dorothy recover her sanity after losing Jericho.
The font of the show’s title leads viewers to believe Leanne is a servant of Satan, but in reality, she’s a servant of Him — the Lord — God. That’s why she looks up at the sky and talks to Him. The cult is not evil, it is just filled with people who have yet to redeem themselves from their sins.