Albert Barnes Commentary John 17:19

"And for their sakes I sanctify myself, that they themselves also may be sanctified in truth." — John 17:19 (ASV)

I sanctify myself. I consecrate myself exclusively to the service of God. The word sanctify does not refer here to personal sanctification, for he had no sin, but to setting himself apart entirely to the work of redemption.

That they also, and so forth.

1. That they might have an example of the proper manner of laboring in the ministry, and might learn from me how to discharge its duties. Ministers will understand their work best when they most faithfully study the example of their great model, the Son of God.
2. That they might be made pure by the effect of my sanctifying myself—that is, that they might be made pure by the shedding of that blood which cleanses from all sin. By this only can people be made holy; and it was because the Savior so sanctified himself, or set himself to this work so unreservedly as to shed his own blood, that any soul can be made pure and fit for the kingdom of God.