The poisoned cup is an emblem of the Catholic saint John the Evangelist. The symbols stems from the popular miracle story whereby at HJohn’s blessing, the poison in a cup of wine intended for his consumption is transformed into a serpent.
Interestingly, the image of a serpent rising from a cup is described in a vision of Isis in the Golden Ass of Apuleius:
“In her left hand was a golden cup, from the top of whose slender handle rose an asp, towering with head erect and its throat distended on both sides. ”
The Poisoned Cup
John transforms the poison