Oh boy here we go. This is something that the game doesn't directly explain. But I feel plenty is implied. I'm going to quote some dialogue from the game and give my interpretation of it. Massive spoilers incoming obviously.

The first thing I want to address is who the source of her divine powers is.
Originally Posted by Netav
Maybe it was Selune all along, that protected her.

I've seen this take, but I don't think it's correct. It is stated in her character sheet that her god is Shar. She does in fact have full protection from the shadow curse even in the deeper areas. It is a bit bugged when you cross the threshold between the lightly and heavily cursed areas. But I have played shadowheart origin and ran around the deepest part of the curse with no issues. She has unique interactions with the star statues ( not having to pass a check to get the blessing), shar speaks to her in the gauntlet, and the narrator hints at her losing shar's favour if you save Aylin.

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Shar empowers her before nightsong, and if she takes the light path she loses that. So where does that leave us with Selūne? This is a line you can say to her deep into act 3 if you're a Selūnite yourself. It sums up my perspective well.

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The moonmaiden, or "she who guides" is not a goddess who likes to directly intervene. But there are some hints that she has not given up on Shadowheart. And is trying to guide her back into the light. Netav has pointed out the dark urge butler comments, but I think the big one is the conversation with Aylin in the Shadowfell. Aylin has been trapped in the Shadowfell since well before Shadowhearts birth. Yet she knows who she is, and not only that, she knows details about her life that Shadowheart herself cannot remember. Here are some Aylin quotes with my interpretation of them.

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^Hinting at her parents fate

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^knowledge about her kidnapping and origin as a Selūnite

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^Promising SH a new life as a Selūnite after she chooses to spare Aylin.

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^Again, Aylin does not seem particularly surprised of SH actions.

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^Aylins last words if SH kills her "shar's child after all". She had faith that she would be set free.

So we have an angelic daughter of Selūne laying eyes on a stranger coming to kill her. She knows more about her life than SH does herself, and has faith that she will be set free. The invisible hand of Selūne is at work here. I can't think of any other reason as to why Aylin would know these things. And those words are extremely effective on Shadowheart, she states that they were the primary reason of why she defied Shar. And to top everything off the game tells you selūne blesses the party when receiving the moonlight glaive.

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Originally Posted by Zentu
Further she has not in ANY WAY proven herself to Selune or the Priesthood and thus should have no or VERY limited power. She should essentially right now be back to level 1 as far as spell access.
Originally Posted by Zentu
From a pure lore/plot point of view there is a work around. Selune wants the Absolute stopped at all costs. To this end she see's fit to make sure Shadowheart's powers stay in tact to help defeat the Absolute.

Here is where I disagree. She was able to overcome decades of conditioning and trauma by rejecting Shar. And in doing so saved a literal angelic daughter of Selūne from imprisonment. Aylin then blesses her, hands her a legendary Selūnite weapon, and sends her on her way. Not to deal with the absolute, but to free herself from Shar's curse and save her parents. If that's not a roaring divine endorsement then I don't know what is. Shadowheart does not need to crawl to Selūne to beg her for a scrap of divine power. Selūne (through Aylin) embraces her.

The ying-yang situation of Shar and Selūne might also make it easier to convert between these deities. Aylin: "Whatever Shar calls her own, Selūne has equal claim to. They are one and the same. Their power is matched and mirrored".

Please let me know your thoughts on this, I'm curious about them.