As many of you may know, before Christianity swept across Europe ‘pagan’ religions were the predominant form of belief and worship. One such pagan belief would be that faeries exist.
Religion plays a large role in how a person views themselves and the world. Even if you are not religious, you grew up in a society based on religious belief and your position for or against religion helps you to place yourself within that society. It also answers some questions about morality, the afterlife, etc. A rule of thumb in casual conversation is to avoid religion and politics because more often than not someone will get angry. Now with this in mind, you could only imagine how difficult it would be for a complete stranger to come into your life and make you change your spiritual beliefs. An ingenious move by the Christian church at the time was to super-impose their belief system on the belief system of the group of people they were trying to convert, in the beginning anyway. People who believed in sub-deities or held a certain set of beliefs or believed certain days to be sacred, all of that was mushed together with Jesus slapped on top. (As many of you may know this is also why we have certain Christian holidays at certain times of the year, like Easter coincided with the solstice, things like that).
So when Christianity came to conquer places like Ireland, where people believe in the fair folk, the priests had to find a way to work that local folk belief into the new religion so that Christianity would gain more followers. What the priests came up with is to say that faeries are angels that have fallen out of favour with God. They were once angels living in heaven, just like the angels that live there now. If you know your angel lore you’ll know that there was a huge battle at some point. It was either a revolt by the angels or an attack by the devil, I’m not sure which, and the faeries couldn’t choose which side they wanted to support. For their indecisiveness they were cast out of heaven, but they couldn’t go to hell as they did not choose to side with the devil, so they were banished to Earth. This might also be where they get their immortality from. Angelology seems like it would be full of symbolism and archetypes and very interesting to learn. If ever you get a chance, look into it.
These are some links to different descriptions of what an angel is