John 17:17 Sanctify them by the truth; your word is truth.
When Jesus was preparing Himself for all He had to endure, He made sure everyone was included in his prayers; here the disciples are targeted.
Jesus wants to make sure the disciples continue to believe in the truth and to share the truth; He also wants things to be written down for others to be able to read the truth.
What a powerful prayer – that we should not just be sanctified but that we should be able to spread the word because of it being written down by men of God at that time!