Puritan belief had it that in order to become a witch, one had to formally sign one’s name in the devil’s book. Through this signature, the witch denounced God and pledged his or her soul to Satan. The latter would then confer supernatural powers upon his new comrade in the war against God. During the Salem witch trials, a confession of having signed the devil’s book was considered key evidence. Read an account of one such confession, that of Mercy Short, here.