“They are not of the world, even as I am not of the world. Sanctify them in the truth. Your word is truth.”
By following Jesus the disciples have removed themselves from the world system and the world's ways. They are no longer tied to the world or absorbed by its interests. Just as Jesus thought only of His Father and his Father's will so was it to be with the disciples. Thus can He pray for them to be sanctified in the truth.
‘Sanctify them in your truth.' To ‘sanctify' means to ‘make holy', to ‘set apart as God's' so that they begin to partake of the ‘wholly other'. They are to have the touch of God on them. Here the power of God's truth within them and around them is to give them that aura of belonging to God. The ‘ Holy Father' (John 17:11) is to make them holy too, ‘in His truth'. It is truth that makes holy, not mysticism. Mysticism without truth can only be dangerous and misleading.
‘Your word is truth.' Jesus has spoken of ‘the word of God' as referring to the Old Testament Scriptures (Mark 7:13 compare Luke 4:4) and elsewhere it refers to the teaching of godly men (Luke 3:2) and especially of Jesus (Luke 5:1; Luke 8:11; Luke 8:21; Luke 11:28) Who is the Word (John 1:1-18). Thus it means God's communication through godly men in whatever form He chooses and especially through Jesus. For us therefore it primarily means the Scriptures as they reveal Him Who is the Word. It is there above all that we can find truth. And it is by absorption of that truth that we become God's own people, set apart to Him and accepted by Him as holy. But no one today has the special unction given to the Apostles. We should beware of any ‘truth' which is not fully based on Scripture taken as a whole and firmly in context.