St. Jude was one of the Twelve Apostles and the brother of St. James, who was also one of the twelve. Jude was described by St. Mathew (13:55) as being one of the "brethren" of Jesus, probably meaning a cousin since the Hebrew word for "brethren" indicates a blood relationship. Elsewhere, Jude's mother, Mary, was referred to as a cousin of Jesus' mother.

St Luke's Gospel includes Jude in the list of the 12 Apostles (6:16) and St. John mentions him (14:22). St. Mathew (10:3) and St. Mark (3:18) use the name Thaddeus without Jude. Catholic scripture scholars have long held that Jude and Thaddeus are the same person, and that is why the Church speaks of St. Jude Thaddeus.