Here it is:
People who know nothing about a subject are radically vulnerable to those who know less than nothing.
— those knowing less than nothing being people who have strongly held but baseless opinions. This law of the universe raises its head almost every time some American journalist or tweeter or YouTuber makes an argument based on a claim about our history. Some fatuous statement becomes a mantra for people who lack (a) the knowledge to test the claim and (b) the initiative to acquire the knowledge.