“Jesus prays, ‘sanctify them’, that is, ‘perfect them and make them holy. And do this in the truth’, that is, in me, your Son, who am the truth (Jn 14:6).
It is like saying: Make them share in my perfection and holiness (sanctity). And thus he adds, ‘your word’, that is, your Word, is the truth. The meaning is then: Sanctify them in me, the truth, because I, your Word, am the truth.
Or, we could say this: Sanctify them, by sending the Holy Spirit. And do this in the truth, that is, the knowledge of the truths of the faith and of your commandments: ‘You will know the truth, and the truth will make you free’ (John 8:32). For we are sanctified by faith and the knowledge of the truth: ‘the righteousness of God through faith in Jesus Christ for all who believe’ (Rm 3:22).
The righteousness of God through faith for all who believe
He adds, ‘your word is truth’, because the truth of God’s words is unmixed with falsity: ‘All the words of my mouth are righteous; there is nothing twisted or crooked in them’ (Pr 8:8). Further, his word teaches the uncreated truth.
Another interpretation: In the Old Testament everything set aside for divine worship was said to be sanctified: ‘Then bring near to you Aaron your brother, and his sons with him, from among the people of Israel to serve me as priests’ (Ex 28:1). Accordingly he says, ‘sanctify them’, that is, set them aside, in truth, that is, to preach your truth, because your word, which they are to preach, is truth.”
– St Thomas Aquinas