Thank you for the question on prophets serving as priests, “Why didn’t the ancient prophets serve as priests at times or vice versa since they both served the other’s functions at times?”
As I thought about your questions, here are some thoughts for your consideration.
A. God designated the Tribe of Levi to be priests before God. The Levitical priesthood was an inherited position from the line of Levi. No other person can serve as priest unless they were from that linage.
Numbers 3 says this, “The LORD said to Moses, 6 “Bring the tribe of Levi and present them to Aaron the priest to assist him. 7 They are to perform duties for him and for the whole community at the tent of meeting by doing the work of the tabernacle. 8 They are to take care of all the furnishings of the tent of meeting, fulfilling the obligations of the Israelites by doing the work of the tabernacle. 9 Give the Levites to Aaron and his sons; they are the Israelites who are to be given wholly to him. 10 Appoint Aaron and his sons to serve as priests; anyone else who approaches the sanctuary is to be put to death.”
11 The LORD also said to Moses, 12 “I have taken the Levites from among the Israelites in place of the first male offspring of every Israelite woman. The Levites are mine, 13 for all the firstborn are mine. When I struck down all the firstborn in Egypt, I set apart for myself every firstborn in Israel, whether human or animal. They are to be mine. I am the LORD” (NIV). See also Numbers 7.
Verse 10 says that anyone else other than Aaron and his sons who approaches the sanctuary is to be put to death. Thus no other person from another tribe can serve as priest before God.
B. The judges were responsible for teaching and the priests would apply the Law for the nation of Israel. The most difficult cases that the judges couldn’t resolve was to bring them to the priest to seek God’s answer.
Exodus 18 says this, “Moses’ father-in-law replied, “What you are doing is not good. 18 You and these people who come to you will only wear yourselves out. The work is too heavy for you; you cannot handle it alone. 19 Listen now to me and I will give you some advice, and may God be with you. You must be the people’s representative before God and bring their disputes to him. 20 Teach them his decrees and instructions, and show them the way they are to live and how they are to behave. 21 But select capable men from all the people—men who fear God, trustworthy men who hate dishonest gain—and appoint them as officials over thousands, hundreds, fifties and tens. 22 Have them serve as judges for the people at all times, but have them bring every difficult case to you; the simple cases they can decide themselves. That will make your load lighter, because they will share it with you. 23 If you do this and God so commands, you will be able to stand the strain, and all these people will go home satisfied” (NIV).
If the judges and priests were righteous in their conduct, there would be no need of prophets. However as one reads Judges, everyone did what was right in their own eyes. The judges and priests were corrupt, even the sons of Eli and Samuel.
C. The rise of prophets were to rebuke and call the people and priests to return to the LORD God.
God chose certain men from the differing tribes to be judges and prophets for the nation of Israel. As one reads Judges, the people drifted away from God. God allowed his people to be invaded whereby they cried out to the LORD God for deliverance. God raised up a man like Gideon, Jethro, even Samson to deliver his people as they repented of their evil ways. In time, God raised up judges-prophets who both proclaimed and prophesied of judgment before the rulers of Israel if the Israelites didn’t repent of their sinful ways.
The prophets didn’t replace the priests since God first appointed and gave the Tribe of Levi the honor to be priest before God. If God allowed other individuals to become priests, then the line of authority and succession would be distorted or confused as anyone can claim to be a priest. That is what the King Jeroboam of the Northern Kingdom did by establishing his own center of worship, leading the people into idolatry (I Kings 12:25–33).
SUMMARY: The role of priest was to make atonement for sins while the role of a prophet was to call rulers and people to repent of their sins. God didn’t establish a lineage of prophets but he did for the priesthood.
For more discussion: