Hierateúō (ἱερατεύω) means to serve as a priest, to perform priestly duties. Related to hiereus (priest) and hieron (temple). Appears in Luke's Gospel.
Luke 1:8: Zechariah was 'serving as priest before God' when Gabriel announced John the Baptist's birth. The priestly ministry was the highest religious duty in Israel. The NT then makes a stunning application: through Christ, all believers are priests (1 Peter 2:5,9; Revelation 1:6). The veil is torn; every believer has direct access to God. Our 'priestly service' is the sacrifice of praise (Hebrews 13:15) and holy living. Every Christian daily exercises a priestly calling.