Why is my step kids biological mom blaming me for her kid losing his faith

My husband isn't religious but he does believe in a higher power. I don't believe in anything. His ex is very religious and their kids go to church every Sunday. I will say my oldest step child has had probably the worst year of his life. He's 17 and we put him in therapy. His mom called me yelling at me asking what I've done to her son. He told his mom he doesn't want to go to church anymore. She is saying I must have been the one poisoning his mind. Religion or God doesn't really ever come up here. I don't talk about my beliefs. My step daughter tells me from time to time what they learned at church and I listen. I am not telling people what to believe. I don't think me and my step son ever had one conversation about religion. He's telling his mom he doesn't believe anymore but that has nothing to do with me. She's threatening to take this to court to demand full custody because we are messing with religion in her home when that's not true.