With his left hand, Jesus' favorite disciple points in a highly expressive manner at the goblet with a snake that he holds in his right hand. This alludes to the poison he was forced to drink while in prison, a test from which he emerged unharmed.

This would seem to be a single work rather than part of an Apostolate, although it is very similar —in its iconography, composition and format— to the representations of Saint John and the Apostolates at the Museum-House of El Greco and at Toledo Cathedral.

Despite the lack of a signature and of any information as to its provenance, the critics are unanimous in accepting this as an original work by El Greco's own hand.