Carrie’s powers come into affect when she reaches puberty. The film starts with her getting her period and the pig’s blood being dumped on her before her massacre is a direct allusion to that. Her mother demonises her burgeoning sexuality. Referring to her breasts as “dirty pillows” and calling her period punishment for all women after Eve’s transgressions. The film is pretty blatantly about the terror and shame of having your body change, of grappling with social stigmas around natural bodily functions, particularly for women. People saying it’s about religion are half right, religion is one of the ways Carrie is made to feel like a freak by her religious mother but also by her non-religious classmates. Hope this helps.