John 17:17 Sanctify them in the truth; your word is truth.
And so we consider from whence that passage comes and what is the word that Jesus says is truth.
Jesus is praying here to the Father for those men whom He has chosen who will be sent as He was sent by the Father to bring others to faith. Jesus is also praying for all those who come to the faith by the words of the Apostles that were given them by Jesus.
The words were orally transmitted by Jesus, not written words and the Word that Jesus gave them was of Himself not written words, not a book, but those which testified of Him.
The Word of Truth is Jesus.