By calling Jesus a Samaritan with a demon, his opponents used a racial insult to dismiss his accusations (see study note on 4:4–6). They claimed Jesus was demon-possessed to counter his claim that they belonged to Satan (also 7:20; 8:52; 10:20; 8:44).
Jesus rightly replied that these words were deeply offensive. In fact, it was a serious and unforgivable offense (Mark 3:22–29).