Sanctify (αγιασον). First aorist active imperative of αγιαζω. To consecrate or set apart persons or things to God. See Exodus 28:41; Exodus 29:1; Exodus 29:36; Exodus 40:13. See Paul's prayer for the Thessalonians (1 Thessalonians 5:23). This is done in the sphere (εν) of truth (God's truth), God's Word (not human speculation, but God's message to us).