Nothing, but I suspect your concept of "religion" is not the same as the parent's. Some people think of religion is a valuable tool humans invented to help them govern their lives . To others, religion is a doctrine that teaches ostensibly facts about the nature of the universe. Typically these facts lead to conclusions such as "humans SHOULD do x, MUST NOT do y", etc.

Two completely different concepts, one word. So it's a bit confusing. But for someone discussing the second type of religion, saying it's "man-made" is definitely a knockout punch.