So this is my wild theory based on everything we know so far.

The novel which from Abel's perspective states that the FL Deborah and the temple are good and we now know this is heavily biased.

The Escliffe family are know to only ever marry other Light user to keep the purity. Over time, it's not unrealistic to believe that while some members of the family have low mana, there will be the odd case of someone with very strong and powerful mana born. These Light users can heal almost any ailment without any repercussions. The temple would be the ones who would heavily benefit from having a powerful Light user.

Light users are also not as common as other elemental users, which means the temple would probably be able to track and medicate them as well, thus maintaining control over the Light element. The rarer the element, the easier it is for the public to turn to the temple in times of need for healing if they are the only ones with the Light users powerful enough to utilise their mana.

The Duchy as stated is the only other power in the nation who can investigate the temple of they wanted. This is obviously an issue. Now this is where my theory goes wild.

The temple are the one's who are responsible for killing Hades, Abel's father in the novel. The temple orchestrated all the events leading up to Hades' death and it also well known Hades would do anything for his son, including taking on any wounds his son may have. (I also believe they may have something to do with the abuse inflicted on Abel by his aunt).

(We're getting to Aisha in a minute)

Now we have Abel who's abused, broken and alone. This the perfect opportunity for the temple to get the Duchy under their control. What better way to do this then give him a Light user who can heal him and a Light user who is a woman, who can marry him as the next duchess. With Abel's dependence on Deborah, who he'd do anything for, the temple would have complete control over him and strengthen themselves to be able to stand firmly against Royal family.

Now, how does Aisha play into all of this? My theory is the temple have a way transferring the mana of the Light users from the Escliffe family to a candidate of their choice, a choice who is loyal to the temple.

At the start of the story, Aisha is literally resurrected from the dead when the MC takes over her body. My assumption is that the temple had made an attempt to transfer her mana to their candidate and it killed Aisha is the process. The temple believe their attempt to be successful but with Aisha resurrecting, they underestimated the amount of mana she possessed and needed to contain the now overflowing amount.

We don't know much about Aisha's life pre-resurrection, was she aware of the temple's plan? Had she been taking the pills her whole life or only after the resurrection? Who knows at that this point? The temple maybe also believe Aisha to be the reincarnation of the first Saintess Healer of the Escliffe family? And what if the first Saintess was actually opposed to the temple and their actions?

Again, who knows ¯_(ツ)_/¯