JT asks about the origin of “Priesthood of the Believer”. That’s a good question because at first glance, it’s a doctrine that would make for a very confused Church.
I personally agree with Syndrome but hold that thought. Officially, the reason for the principle is 1 Peter 2:4-9:
And coming to Him as to a living stone which has been rejected by men, but is choice and precious in the sight of God, you also, as living stones, are being built up as a spiritual house for a holy priesthood, to offer up spiritual sacrifices acceptable to God through Jesus Christ. For this is contained in Scripture:
“Behold, I lay in Zion a choice stone, a precious corner stone,
And he who believes in Him will not be disappointed.”
This precious value, then, is for you who believe; but for those who disbelieve,
“The stone which the builders rejected,
This became the very corner stone,”
and, “A stone of stumbling and a rock of offense”; for they stumble because they are disobedient to the word, and to this doom they were also appointed.
But you are a chosen race, a royal priesthood, a holy nation, a people for God’s own possession, so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light; for you once were not a people, but now you are the people of God; you had not received mercy, but now you have received mercy.
Building on that, priests are defined by several traits:
1. Direct access to God. Ephesians 3:12, This was in accordance with the eternal purpose which [God] carried out in Christ Jesus our Lord, in whom we have boldness and confident access through faith in Him. Also, the tearing of the curtain in the Temple at the moment of Christ’s death.
2. Offer sacrifices. Romans 12:1 Therefore I urge you, brethren, by the mercies of God, to present your bodies a living and holy sacrifice, acceptable to God, which is your spiritual service of worship.
3. Mediate between God and men. Most of everything the Apostles said and did and told us to do.
Lastly, before the time of Christ the Holy Spirit was only given to selected members of society, chiefly Judges, Levites and prophets. But in NT times, the Holy Spirit is given to all believers. Christ declared this in John 14, it was realized at Pentecost and it was prophecied in Joel 2:28-29:
It will come about after this
That I will pour out My Spirit on all mankind;
And your sons and daughters will prophesy,
Your old men will dream dreams,
Your young men will see visions.
Even on the male and female servants
I will pour out My Spirit in those days.
So, all believers now have the privileges and duties of priests, therefore we have “priesthood of the believer”.
Now that the official explanation is out of the way, I think it’s incorrect. What we actually have today is Priesthood of Christ. While He has not taken His throne, Christ is active today as humanity’s high priest. The book of Hebrews has much to say about this, particularly Heb. 3:1-6:
Therefore, holy brethren, partakers of a heavenly calling, consider Jesus, the Apostle and High Priest of our confession … Now Moses was faithful in all His house as a servant, for a testimony of those things which were to be spoken later; but Christ was faithful as a Son over His house—whose house we are, if we hold fast our confidence and the boast of our hope firm until the end.
And Heb. 4:14-16:
Therefore, since we have a great high priest who has passed through the heavens, Jesus the Son of God, let us hold fast our confession. For we do not have a high priest who cannot sympathize with our weaknesses, but One who has been tempted in all things as we are, yet without sin. Therefore let us draw near with confidence to the throne of grace, so that we may receive mercy and find grace to help in time of need.
Christ is our active High Priest, therefore we no longer need a human intermediary… not even each other. The official Prot position isn’t wrong so much as it ignores the fact that Christ is still active in the world, even if only by proxy of His Spirit. We become like priests only because God conforms us to be like Christ, the Priest. No believer is a substitute for Him. This view also avoids the appearance of anarchy/disunity that I suspect moves many Catholics against Protestantism. We aren’t all priests, we don’t each decide for ourselves what God wants. Christ told us what God wants and He’s still active in our affairs. (John 14)
On a closing note, there’s still good reasons to have official clergy. Affairs of the Church won’t handle themselves, as the Apostles found out and elevated Stephen to handle. False doctrines must be confronted, missionaries supported, disaster relief moved about. And it’s nice to have a designated, Godly man available for life’s major (and frequently sudden) events.
But none of that requires a specially trained professional who gets a privileged hotline to the Big Man. To the contrary, “Elijah was a man just like us.” James 5:17.