"In John 17:17 our Lord does not say, 'Thy word is truth', He remarks rather, 'Thy word is truth'. Earlier in the Gospel of John He had indeed identified Himself as the truth [14:6] 'I am...the truth', were His words. In the present passage, however, when He is not speaking of Himself, but the Word of God, He characterizes that Word as true. Christ is Himself the Truth, and when He speaks of Himself, there is need for Him to employ the definite article. In the prayer, however, He merely intends to characterize the Word of God as truth. 'The Word of God', we may paraphrase, is true; it speaks the truth; the message which it offers is a true message. It is dependable; it is truth. The Word of which He speaks is the message of God; it is information which God has communicated to the world; it is that which God has spoken. The word of God, which Jesus Christ Himself has spoken, is truth."- E.J. Young
Source: Young, E.J. Thy Word Is Truth - Some Thoughts on the Biblical Doctrine of Inspiration. reprinted 2008. Pennsylvania: Banner of Truth, 1957. p. 262.