Didymus was the Greek name used for the apostle Thomas, and it means “twin.” The New Testament preserves both names, presenting Thomas as “Didymus,” the Greek equivalent of the Aramaic name T’oma or Thomas.
Thomas was one of the Twelve Apostles. The Gospel of John uses the Greek term Didymus when describing him, including the scene in which he initially doubts that the risen Jesus has appeared to the other disciples. After seeing Jesus’s wounds, Thomas declares, “My Lord and my God.”
The name Thomas itself comes from an Aramaic word meaning “twin,” related to the Hebrew term for twin. Thus Didymus is not a separate apostle or a surname; it is a translation or explanatory equivalent of Thomas.
Later Christian and Syriac traditions sometimes call him Judas Thomas, and apocryphal writings expand on the identity of his supposed twin. Those traditions should not be confused with the straightforward New Testament use of Didymus.