How do you respond to the claim that Christians are narrow-minded bigots? The person hasn’t said anything about your claim or your argument. They are simply calling you a name.
Explain to them that name-calling is not a good way to argue for their position or to argue against mine.
Ask a question, “What do you mean by narrow-minded?” They think they’re right, that’s why they’re correcting you. By their own definition, they are narrow-minded too.
Just because someone makes a narrow claim does not mean they are narrow-minded. Furthermore, it is illegitimate to dismiss someone by simply calling him or her a name.