If they are a typical Christian, their theology has falsifiable claims. Unless they are some kind of Unitarian. So, it does not require faith to know they are wrong about their faith.
You appear to have a great deal of faith in your own ability to read what’s happening in another’s mind and heart. Eg… you have no idea what elements of any particular religion a person considers relevant to their memberships.
If they are a typical Christian, their theology has falsifiable claims. Unless they are some kind of Unitarian. So, it does not require faith to know they are wrong about their faith.
You appear to have a great deal of faith in your own ability to read what’s happening in another’s mind and heart. Eg… you have no idea what elements of any particular religion a person considers relevant to their memberships.