Eli’s mother was unable to conceive a child, so she prayed to Satan in order to get pregnant. The devil promised it would be totally chill to give birth to the spawn of Satan, but as Rose notes in the film, the devil always lies. (Had Rose done a little bit of research, she would have realized quickly that this was, in fact, an incredibly stupid plan.) Rose kept Eli away from the world so that he wouldn’t hurt anyone as his powers progressed, in hopes that she could one day have Eli “fixed” of the problems caused by his disturbing parentage. The treatments given to Eli by Dr. Horn — a nun as well as a doctor — were just infusions of holy water, which caused Eli immense pain but suppressed his evil powers.