William Burkitt on John 17:19

The word sanctify here, is not to be taken for the cleansing, purifying, or making holy, that which before was unclean; but Christ's sanctifying himself imports,

1. His separation of setting himself apart to be a sacrifice for sin.

2. His consecration or dedication of himself to this holy use and service.

Hence learn, that Jesus Christ did dedicate and solemnly set himself apart to the great work and office of a Mediator.

Learn, 2. That the great end for which Christ did thus sanctify himself, was, that he might sanctify his members; therefore did he consecrate and set himself apart for us, that we should be consecrated to, and wholly set apart for, him.