15 Now Simon Peter and another disciple followed Jesus. That other disciple was known to the high priest, and he went in with Jesus to the court of the high priest.
16 Now Peter stood outside the door. The other disciple, the one known to the high priest, went out and spoke to the female door-keeper and brought Peter in.
Who was the other disciple?
There have been a number of suggestions as to his identity.
1 Some have suggested that it may have been Nicodemus. We know that he secretly admired Jesus and had spoken to Him about His teaching. He said this:
Rabbi, we know that You are a teacher come from God, because no one could do the signs which You do if God is not with Him – John 3.2.
Since Nicodemus was the teacher in Israel we can assume that he was well known to Caiaphas & Annas and would be free to enter the court yard of the High Priest.
2 Another suggestion is that the other disciple was Joseph of Arimathaea. Matthew records that he was a disciple of Jesus:
 As evening approached, Joseph, a rich man from Arimathea who was one of Jesus’ followers,
 went to Pilate and asked for Jesus’ body. And Pilate issued an order to release it to him – Matthew 27.57-58 (NLT).
W Barclay makes the point that both of these men were members of the Sanhedrin, the Jewish ruling council. Therefore, they would both have been known to the high priest, and were in Jerusalem for the Passover.
3 A further suggestion is that this ‘other disciple’ might have been Judas Iscariot, since he would have become well known to the high priest and his staff through his dealing with them to betray Jesus. However, it is highly unlikely that Peter would have been in his company after Judas’ betrayal of Jesus in the garden.
4 The most popular suggestion is that the ‘other disciple’ was John and that he was known to the high priest through his trade as a fisherman. W Barclay makes the point that John’s father, Zebedee, had a salt fishing business that was sufficiently wealthy enough to employ others. This may have meant that he was able to transport salt fish from Galilee to Jerusalem, and so supply the household of Caiaphas, the high priest. If that was so, then John may have been involved in such transportation and would have been know to the high priest and his staff.
That John was the ‘other disciple’ is generally accepted, but we cannot be absolutely certain.