Anthony Grafton claims that "the Christian discovery of a Jewish Jesus began not in the 19th century but in the Renaissance"; elsewhere, "Sixteenth and seventeenth-century scholars came to see, as clearly as contemporary specialists on the New Testament, that Christianity began as a Jewish sect". 2
I didn't watch the entire lecture, but I wasn't even aware that there was any point in history where it was believed that he wasn't Jewish. He's referred to as "King of the Jews" in the New Testament, after all. I couldn't find any sources discussing this; does anyone know of any?