Continuity mistake: When Sister Irene's habit is ripped at the back, you can see a bra strap on her left shoulder which disappears when she is seen at the front and when the camera is closer to her back (when she has the pentagram carved into her).
Visible crew/equipment: When water blurs the Father's word book, and the word deliveries appears, a woman appears to the right of him that appears to be a member of the camera crew, or possibly Irene, but she shouldn't be there either, because she is praying with imaginable nuns at that time. (01:06:22)
Continuity mistake: Irene puts red paint on the kid's nose which disappears when they all turn around. (00:10:00)
Other mistake: Sister Irene digs father Burke out of his grave in a crisp white night gown but doesn't get any dirt on it.
Continuity mistake: When Sister Irene and Father Burke are eating after he was buried alive, Irene's fork keep changing from in her hand to resting on the plate between shots.
Character mistake: If the nuns in the Abbey weren't real and were created by Valek, why did Valek get one of the nuns to explain to Sister Irene the exact way to defeat her?
Other mistake: Father Burke is buried in a coffin that is not air tight. Light from his torch can be seen outside the coffin. Yet no dust or dirt works its way into the coffin. (00:40:00)
Continuity mistake: When Frenchie drops off food in the beginning, the cold cellar door opens on the right, but when he takes the priest and Sister Irene there it opens on the left.
Continuity mistake: Spoilers: At the end we are shown how this film connects into The Conjuring and shown a scene from that film. However the clip in the nun is different to The Conjuring. Ed Warren has an extra line for example.
Character mistake: When the father gets buried alive, he rings the bell. Then, when the (almost) nun hears the bell and comes running, you'll notice she calls out for the father when there's no way she can know it is him by just the sound of that bell. (00:40:24)
Answer: The nun was trying to find the relic. The deceased nuns had no actual idea where it was. That's the whole reason Irene was summoned by the Vatican because she had visions about "Mary pointing the way" also the "ghost nuns" more than likely have something to do with her clairvoyance as well.