In the book of Genesis, after punishing him for murdering Abel, God places an unspecified mark on Cain to protect him from harm. Eva Mroczek notes some of the varying interpretations of this passage:
The Bible connects the mark with divine protection, but some interpreters link it with the curse that God placed upon Cain, imagining it as a badge of shame. One suggestion in a Jewish midrash, for example, is that Cain was punished with leprosy. . . .
The mark of Cain has also been interpreted in anti-Semitic ways. Some Christian interpreters saw Cain as the prototype of the Jewish people (although, according to the biblical genealogy in Genesis 5, Jews are not Cain’s descendants—nor, for that matter, is anyone else, as it was Noah’s family, descended from Seth, who survived the flood). Saint Augustine (354–430) connected the “mark of Cain” to the observance of Jewish law: the Jews “never lost the sign of their law, by which they are distinguished from all other nations and peoples,” whereas another Christian theologian, Isidore of Seville (560–636), linked it more precisely with circumcision. Extrapolating from these motifs, other Christian readers imagined Cain according to offensive stereotypes of Jews—with a hooked nose or horns, distinct in appearance and condemned to endless wandering. . . .
But other interpreters did read the mark as something protective. The same midrash that mentions leprosy also suggests a range of other possibilities: for instance, it relates that Cain grew a horn, which is both a mark of identity and a defensive weapon. This same text also speculates that God gave Cain a dog as the “mark.” Though dogs tend to be portrayed negatively in classical Jewish sources, the dog might be a sign both of stigma and of protection from attackers.
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