Plunged Into Boiling Oil, Yet Unharmed

Recently came across this quote from Tertullian (AD 155-230) concerning the Apostle John:

Since, moreover, you are close upon Italy, you have Rome, from which there comes even into our own hands the very authority (of apostles themselves). How happy is its church, on which apostles poured forth all their doctrine along with their blood! Where Peter endures a passion like his Lord’s! Where Paul wins his crown in a death like John’s! Where the Apostle John was first plunged, unhurt, into boiling oil, and thence remitted to his island-exile! — from The Ante-Nicene Fathers, Volume 3, "The Prescription Against Heretics," chapter 36.

Tertullian tells us that John was plunged into boiling oil and yet was unhurt.