Mary Lou loves her children and wants to keep them from the fire of hell. But she can't ignore that they are getting older, and she has caught Chastity flirting more than once. She wants to save all her children. But it is better for one to fall than for that fate to befall all of them.

She arranges for Credence to be raped to form the basis of a sermon about purity and the undesirability of sex. Making credence tell the others what he experienced and explaining to her children how sinful credence is.

Tldr: Mary Lou's terrible parenting includes Credence being raped so she can use him as a morality tale