Why do Matthew’s and Mark’s listings of the names of apostles differ from Luke’s in listing Thaddeus instead of a second Judas?
The apostle Thaddeus mentioned in the Gospels of Matthew (10:3) and Mark (3:18) is one and the same as the first Judas mentioned in Luke (6:16). It is likely that this apostle was called by other names to avoid confusing him with Judas Iscariot. A Catholic Commentary on Holy Scripture explains:
Doubtless like others of his time he enjoyed several names, and it is natural that the early Christians should have avoided the name he shared with the traitor. (p. 948)