More genital mutilation of babies happens in America than any other country on earth. Think about that for a moment.
It is about marking you as subservient. If you read anything about Evangelical Christian culture the single worst sin you can ever commit is demanding your autonomy. This grievous violation of our bodies is used to induce us into servitude. I reject the marks this wicked culture has given me.
That entire line is garbage and meant to support the subservience, exactly as you've said. A mother's strongest instinct is to protect her children, so the tribe taking a child and cutting his body marks him as belonging to the tribe rather than to the family. It proves, in plain view, that the familial bond is secondary, and individual autonomy serves at the pleasure of the group, not the other way around.
Circumcision is explicitly not a Christian practice, though. It's done by many Americans (who happen to be evangelical) for cultural reasons, but the Christian Bible literally is explicitly against circumcision being done by Christians for spiritual or religious reasons.