The entire film is a religious allegory.
Him allows the older couple into his house because they are fans of his work as a poet. The couple's sons arrive and start a fight that leads to the older son killing his brother. After becoming frustrated with the number of guests showing up, Mother and Him have sex, leading to her pregnancy.
Near the end of Mother's pregnancy, a massive number of Him's fans show up at the house, quickly growing violent and erratic. Mother gives birth to a son, only to have her baby taken away and killed/eaten by the crowd. After the crowd beats her, Mother goes into the basement to ignite the oil tank and destroy everyone in the house, herself included. Him removes her heart to pull out a crystal object that he uses to "bring life into the house". A younger wife appears on their bed, indicating that the cycle is set to repeat itself.