In the Christian New Testament, Joseph Justus (also known as Barsabbas) figures momentarily in the casting of lots among the 120 or so gathered together after the Ascension of Jesus, to replace Judas Iscariot and bring the Apostles again to the number twelve. According to Acts of the Apostles i.23 – 26:

23.So they proposed two men: Joseph called Barsabbas (also known as Justus) and Matthias. 24.Then they prayed, "Lord, you know everyone's heart. Show us which of these two you have chosen 25.to take over this apostolic ministry, which Judas left to go where he belongs." 26.Then they cast lots, and the lot fell to Matthias; so he was added to the eleven apostles."

Since the passage identifies the candidates as "of these men which have companied with us all the time that the Lord Jesus went in and out among us, beginning from the baptism of John, unto that same day that he was taken up from us", Joseph was a member of the wider circle of disciples.

Further identification of this shadowy figure is insecure, though in Christian tradition, he is numbered among the Seventy Disciples mentioned in Luke 10:1-24. No names are given among the Seventy-Two or Seventy Apostles of Christ—depending on the manuscripts—that were sent out in Luke 10.1:"two by two, into every town and place where he himself was about to go." Nor is this "Joseph the Just, son of the 'sabbath'"—as his name translates— identified as one of the adelphoi of Jesus— however this Greek word for "brothers" may be interpreted—in this one passage in Acts in which he appears. Nor does such a "Joseph Justus" appear among the disciples in the lists given in the synoptic gospels, unless he were represented unrecognized in some way under an alias. It would appear to be an inexplicable choice made by Peter among the assembled adelphoi (Acts 1.15, translated in the New International Version "believers").

Judas Iscariot (Hebrew: יהודה איש־קריות‎, Yehuda, Yəhûḏāh ʾΚ-qrayyôṯ) was, according to the New Testament, one of the Twelve Apostles of Jesus. He is infamously known for his kiss and betrayal of Jesus to the hands of the chief Sanhedrin priests for a ransom of 30 pieces of silver.

In the Greek New Testament, Judas is called Ιούδας Ισκάριωθ and Ισκαριώτης . "Judas" (spelled "Ioudas" in ancient Greek and "Iudas" in Latin, pronounced yudas in both) is the Greek form of the common name Judah (יהודה, Y^ehûdâh, Hebrew for "God is praised"). The Greek spelling underlies other names in the New Testament that are traditionally rendered differently in English: Judah and Jude.

The significance of "Iscariot" is uncertain. There are several major theories on etymology:

Judas is mentioned in the synoptic gospels, the Gospel of John and at the beginning of Acts of the Apostles.

Mark states that the chief priests were looking for a sly way to arrest Jesus. They decided not to do so during the feast since they were afraid that people would riot; instead, they chose the night before the feast to arrest him. In the Gospel of Luke, Satan enters Judas at this time.