John 17:19

And for their sakes I sanctify myself
Which is to be understood, not of his making himself holy; for he never was a sinner, and so stood in no need of sanctification: he was made like unto us, yet without sin; he looked like a sinner, but was not one; he was traduced, charged, and treated as such, but was perfectly holy, and free from all sin; he was essentially and infinitely holy as God; and as man, he was holy in his conception and birth; he was filled with the Holy Ghost, and was holy in his life and in his death: rather this may be meant of his being separated, and set apart for his office as Mediator, which, though done by the Father, and is ascribed unto him, ( John 10:36 ) ; yet may also be attributed to himself; since he voluntarily devoted himself to this work, and cheerfully accepted of it: though it seems best to understand it of his offering himself a sacrifice for, and in the room and stead of his people, in allusion to the offerings under the law, the sacrificing of which is expressed by sanctifying, ( Exodus 13:2 Exodus 13:14 Exodus 13:15 ) ; and because his sacrifice was an Holy One: what he sanctified or offered was "himself": not his divine, but human nature, his body and his soul; and these as in union with his divine person; which gives his sacrifice the preference to all others, and is the true reason of its virtue and efficacy; and this is expressive of his great love. He himself is also the sanctifier or offerer, which shows him to be a priest, and that he had a power over his own life, and that he sacrificed it voluntarily; and this he is said to do at that present time, because the time was very near that he was to be offered up, and his present prayer and intercession were a part of his priestly office. This he did not for his own sake, nor for the sake of angels, nor for all men, but for his disciples, as distinct from the world; and not for the apostles only, but for all that the Father had given to him; and that as their substitute and surety, in their room and stead:

that they also might be sanctified through the truth;
that is, have all their sins expiated, and they be cleansed from all the guilt and filth of them, through Christ himself and his sacrifice, who is the truth; or "in truth"; as it may be rendered, really and truly, in opposition to the legal sacrifices which atoned for sin, not really, only typically; or through the. Gospel of truth, bringing the good news of atonement by the blood and sacrifice of Christ, and which the Spirit of God seals to the conscience with comfort and joy.