Christ, in His high priestly prayer, asks the Father to sanctify His people in the truth. To sanctify is to make holy. To be holy is to be set apart, to be "other than.” Only God is intrinsically holy (separate from all creation, eternal, self-existent, infinite, etc.). It is God’s manifested presence that renders His creation holy. We become holy when we are reconciled to God and set apart unto His service to live for His glory. Jesus is praying that the holy Father would make us holy.