You are strongly recommended to add to your library the excellent revised edition of Harper's Bible Dictionary titled, The Harper Collins Bible Dictionary, Revised Edition [book review], edited by Paul J. Achtemeier, with the Society of Biblical Literature (NY: Harper Collins, 1996). It is currently the best one-volume Bible dictionary in English, and it is available at Border's Books, Christian Science Reading Rooms, http://www.borders.com, or http://www.christianbook.com.

Judas, the son of James and one of the apostles (Luke 6:16; Acts 1:13; John 14:22). This is probably the Thaddaeus of Matt. 10:3; Mark 3:18. The qualification of Judas in these latter two passages only makes sense if there were another Judas in the group.

Thaddaeus, one of the twelve apostles as identified in two of the apostolic lists (Matt. 10:3; Mark 3:18). In the other lists, however, ‘Judas, the son (or brother) of James,’ appears instead of ‘Thaddaeus’ (Luke 6:16; Acts 1:13). Variant readings in Matthew have ‘Lebbaeus,’ and the kjv reads ‘Lebbaeus, whose surname was Thaddaeus.’ ‘Lebbaeus,’ however, does not appear in the best texts and is omitted in most translations. Resulting explanations include: (1) Thaddaeus, Judas, and Lebbaeus are the same person; (2) the lists in Luke and Acts reflect a change in the apostles; and (3) tradition has preserved the fact that Jesus had an inner group of trusted followers (the Twelve) and has retained the names of most of the members of this group but not without slight variations. The second explanation appears unlikely. The third is plausible in light of the fact that the Gospel traditions were transmitted within different communities, making some variation far from surprising. Those who argue that Thaddaeus and Judas are the same person still question his identification with the author of the Letter of Jude. Post-biblical traditions supply additional information about Thaddaeus, but this is of questionable value.