Definition and Origin
Priestly duty refers to the responsibilities and functions assigned to the priests in the religious context of ancient Israel, as outlined in the Old Testament. The role of the priest was established by God through Moses, with Aaron and his descendants being appointed as the first priests (Exodus 28:1). The priesthood was a sacred office, and those who served were required to adhere to specific laws and rituals to maintain the sanctity of their service.

Functions and Responsibilities
The primary duties of the priests included offering sacrifices, maintaining the Tabernacle (and later the Temple), teaching the Law, and acting as mediators between God and the people. The Book of Leviticus provides detailed instructions on the various offerings and sacrifices the priests were to perform, such as burnt offerings, grain offerings, peace offerings, sin offerings, and guilt offerings (Leviticus 1-7).

Priests were also responsible for the daily maintenance of the Tabernacle, which included lighting the lamps, burning incense, and replacing the showbread (Exodus 30:7-8; Leviticus 24:5-9). They were tasked with ensuring that the sacred space remained holy and that the rituals were conducted according to God's commandments.

Teaching and Judging
In addition to their ritual duties, priests were charged with teaching the Israelites God's laws and statutes. They served as judges in legal matters, particularly those involving religious or ceremonial law (Deuteronomy 17:8-11). The priests were expected to possess a deep understanding of the Law and to guide the people in living according to God's will.

Holiness and Consecration
The priests were required to maintain a high standard of holiness, as they were set apart for service to God. This included adhering to specific purity laws and undergoing consecration rituals before assuming their duties (Leviticus 8-9). The high priest, in particular, had additional responsibilities and privileges, such as entering the Holy of Holies once a year on the Day of Atonement to make atonement for the sins of the people (Leviticus 16).

The Priesthood of Believers
In the New Testament, the concept of priestly duty is expanded to include all believers in Christ. The Apostle Peter writes, "But you are a chosen people, a royal priesthood, a holy nation, a people for God’s own possession, to proclaim the virtues of Him who called you out of darkness into His marvelous light" (1 Peter 2:9). This passage emphasizes the spiritual priesthood of all Christians, who are called to offer spiritual sacrifices and to live lives that reflect God's holiness.

Christ as the High Priest
The Book of Hebrews presents Jesus Christ as the ultimate High Priest, who fulfills and surpasses the Old Testament priesthood. Christ's sacrificial death and resurrection provide the perfect atonement for sin, rendering the old sacrificial system obsolete (Hebrews 7:27). As the High Priest, Jesus mediates a new covenant between God and humanity, granting believers direct access to God (Hebrews 4:14-16).

Conclusion
Priestly duty in the biblical context encompasses a range of responsibilities centered on worship, teaching, and maintaining holiness. While the Old Testament priesthood was specific to the descendants of Aaron, the New Testament broadens the concept to include all believers, with Jesus Christ as the ultimate High Priest.