Hell has been obsessed with the retrieval of the Halo since the creation of the first Warrior Nun, and in Episode Ten, Lilith — recently returned from Hell — calls the recently released Adriel a thief. It's an angel's halo, alright, but what if the angel in question isn't Adriel but rather Lucifer?
Also explains why he was carrying the Halo and used it as a weapon in flashbacks instead of simply having it a part of him, he could not insert it in himself because it rejected him so he shoves it in a nearby human to hide it from Hell until he can think of a new plan, but he gets trapped in a tomb instead.
Lilith Has Always Been a Demon
Not getting into the Meaningful Name (Lilith in some Jewish and Christian folklore is the first demon, or the mother of demons, depending on the lore), it may be possible that the Lilith of the show is based in part on the Lilith the Demon Princess of the comics, who represented the lighter, gentler side of Hell. Lilith has a remarkably human-looking body, and she had no special grudge against God or humanity because she was born after the fallen angels were cast out.
Lilith Merged With The Tarask
When she got sucked into Hell with it she ended up merging with it, that's why when she returns her new abilities seem to be similar to the Tarask. The Tarask main weapon is a giant claw, Lilith can grow her nails now into claws sharp enough to slice flesh. She has shown to be able to teleport in a manner that looks just like the Tarask as well and her wound glows with that same hellfire colors the Tarask has.
Lilith also seems to have had a bit of a paradigm shift that could be explained by becoming fused with the Tarask - since coming back, she is drawn to Ava (who has the Halo), knows where Adriel ('the door') is without being told, somehow knows that he is too dangerous to let out, and is aware that Adriel is 'just a thief'. All things that the Tarask, as guardians of the Halo, would have known.
Father Vincent caused the accident that killed Ava's mother
When Father Vincent looks at the book with Ava's name and age he says in response to Lilith that this she is "not entirely random" indicating he knows something about her past. Considering Father Vincent's past as a cartel enforcer we don't know when he became a priest but it seemed like he only did so fairly recently compared to most. We know that he is a recovering alcoholic as well it is possible he was driving the car that hit Ava and her mother and was drunk thus causing the accident that paralyzed Ava and killed her mother. The guilt from that accident making him become a priest.
It may be possible. Vincent mentions to Jillian Salvius that he had been sober for "twelve years, six months and four days", which roughly corresponds to the time of the accident.
Ava had always been the one meant to release Adriel
We know that Vincent was the one behind Sister Shannon's assassination, and we know that he had been in contact with Adriel the entire time through the Divinium hidden under his tattoos. Also, when Lilith mentions that the Halo is in "some random formerly dead civilian," Vincent comments that Ava was "not entirely random." How would he know that? Perhaps he was aware of the murders committed by Sister Frances at the orphanage and made arrangements for the deceased Ava to succeed Sister Shannon so that he could eventually manipulate her into releasing his master. And why doesn't he try to convince Lilith, seeing as she was next in line? Because it would be easier for Vincent to guide an outsider who knows nothing about anything. Also, if it came to it, it would be easier to take the Halo from someone who can barely fend for herself than it would a highly trained ninja nun repeatedly said to be the best in the OCS.
The Tarasks are angels.
Given what we know about Adriel now and their hostility towards him combined with how angels in the bible are far from the "pretty humans" we tend to see in other media, it makes sense.
Heaven and hell are simply human terms for places on 'the other side', not necessarily seperate dimensions, and beings from one side cannot die in the other.
This is why Michael will be fine on 'the other side', and why Adriel can't die in our world.
The identity of Ava's father will become important as the series progresses.
Nobody knows who her dad was, he might make an appearance later on.
The Halo didn't reject Mother Superion, she rejected it.
She theorizes that it may have been her use of its power on Shannon or her recklessness getting the former Mother Superion killed that caused it to reject her, but it doesn't fall out until well after both of those things have happened, and only after the fight is over and the reality of what's happened has sunk in. Although she seems pretty certain that it rejected her, it's not unthinkable that her feelings of guilt in that moment could have caused her to believe herself so unworthy of being the Halo-Bearer that she unconsciously rejected it.
Reya will be the new threat in Season 3.
Given what Adriel said of Reya, she might turn her attention to Earth after seeing their praise of Adriel. This would give Ava ample reason to return and tell the OCS, setting the stage for another grand conflict.
It's possible. Before she vanishes, Lilith warns Beatrice of a coming 'holy war,' so this may be what she is referring to.
Camila has a crush on Ava (or had one on Sister Shannon).
Camila's words to Beatrice that "it's easy to fall in love with a Warrior Nun" suggests that she speaks from experience and as the rookie of the OCS (except for Ava, of course) that would mean the only Warrior Nuns she'd have known personally would be Sister Shannon and Ava. Does Camila hold a torch for Ava or did she hold one for Shannon, perhaps?
The Halo will reach its full potential in Season Three.
Although one cause is the machinations of Adriel, according to Word of God, the main reason why the Halo frequently shorts out on Ava is that she was already dead when it was bonded to her. Unlike with other Warrior Nuns, the power source is always on and is working 24/7 to keep Ava alive and ambulatory, so when Ava uses the Halo's power it is already operating at a deficiency and she can burn it out at a much faster rate. However, assuming that Ava recovers from her injuries on the other side with Reya, the damage may be repaired to the point where Ava no longer needs the Halo to remain alive and kicking. Which could mean that the Halo may have the power to reach its full potential, now that it is no longer being drained by keeping a dead girl alive.
The Halo belongs to Reya.
We know the Turasks have been chasing the Halo-Bearer for centuries from what we're told in season 1, and in the finale of season 2, we see Reya controlling them. Put them together, and it would appear they've been looking for the Halo on her behalf.
