im not religious, but im somewhat open to it, more because i dont feel like we can explain where we came from, how the world came to be, how the universe was formed, and i dont think we ever will. we have some very good theories that make a lot of sense. but we will never be able to prove it, just like you will never be able to prove religion, you have to have faith in either one, science or religion. that and science keeps being proven wrong as we learn more, theories change and are proven wrong all the time. all we can do is keep coming up with new theories to explain everything but we still will never know 100% for sure.
the universe is so insanely complex and hard to understand, we as humans can never understand it completely, we will never have an explanation, science will never be 100% right, and for the same reasons religion will never be the answer. we just cant disprove either completely because we are nowhere close to having the ability to.
there are so many possibilities for forces and things we cant see or know about, there may be a god, maybe its aliens controlling us, or some other force that over time we have felt but couldnt explain so we came up with our different religions to attempt to explain it.
its easy to say something like this video is "some mental illness" for all we know it is. it could actually be the devil possessing that girl, or it could be something beyond science, mental illness, and religion, and be some force we dont know about.
my take on this is that that girl, as well as other exorcism footage ive seen, seems to really be a possessed body, and basically showing true evil. another thing you realize is that almost all the exorcism videos ive seen, the possessed person almost always sounds the same, has that pure evil yelling screaming voice. that even more me would be impossible to make, a girl seems even more impossible. and speaking in different languages which is basically impossible for someone who doesnt know them makes it even more convincing.
my best attempt at an explanation would be that she is possessed by something, its something other than a mental illness. it could be the devil or it could be something else we cant explain, but it seems to me that something separate from herself was making her do that.