Why was Pilate’s statement, “What is truth?” (18:38) included in John’s Gospel?
In John 21:25 John wrote that Jesus did much more than what was recorded in his Gospel. Thus, John was selective in what he included. John gives the purpose of his Gospel as:
Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. (John 20:30–31, RSV)
Thus, one would expect the inclusion of Pilate’s question to relate to this purpose. First, what did Pilate mean by his question, “What is truth?” Initially it sounds like a question expressing truth as relative. However, what Pilate found out questioning Jesus was inconsistent with what the Jewish leaders were telling him.
The Jewish leaders (specifically the Sadducees) apparently did look at Jesus as potentially leading a revolt against Rome:
But one of them, Caiaphas, who was high priest that year, said to them, “You know nothing at all. Nor do you understand that it is better for you that one man should die for the people, not that the whole nation should perish.” (John 11:49–50, ESV)
Thus, they expressed this believed danger to Pilate:
If you release this man, you are not Caesar’s friend. Everyone who makes himself a king opposes Caesar.” (John 19:12, ESV)
But, Jesus’ convincing testimony was that he was no threat to Rome:
My kingdom is not of this world. If my kingdom were of this world, my servants would have been fighting, that I might not be delivered over to the Jews. But my kingdom is not from the world.” (John 18:36, ESV)
Does Pilates question, “What is the truth?” support Jesus being the Christ?
P.S. Further clarification of this question: Truth is an important theme in John's Gospel, but he could have easily ended with:
For this purpose I was born and for this purpose I have come into the world—to bear witness to the truth. Everyone who is of the truth listens to my voice.” (John 18:37, ESV)
leaving out Pilate's question. Pilate's question makes an abrupt and open-ended ending to the conversation. As common in Jesus' conversations with people in the Gospel of John, Pilate seems to side-track from Jesus' spiritual meaning to concerns about his present situation. But, here Jesus doesn't address this question.
It may have no relevance to this question, but note the similarity:
My sheep hear my voice, and I know them, and they follow me. (John 10:27, ESV)