Are all believers priests or just some? These are quotes on the priesthood of the believer vs. the priesthood of the clergy from throughout Church history.
The prescription that the twelve bells [Ex. 28:33, though Scripture doesn't give the number of bells] be attached to the [robe] of the high priest, which hung down to the feet, was a symbol of the twelve apostles. They depend on the power of Christ, who is the eternal Priest, and it is through their voice that all the earth has been filled with the glory and grace of God and of his Christ. Therefore, David also say, "Their sound has gone forth into all the earth, and their words to the ends of the world" [Ps. 19:4]. (Dialogue with Trypho 42)
The priest is not made. He must be born a priest; must inherit his office. I refer to the new birth—the birth of water and the Spirit. Thus all Christians must became priests, children of God and co-heirs with Christ the Most High Priest.
Men universally consider the title of priest glorious and honorable; it is acceptable to everyone. But the duties and the sacrifice of the office are rarely accepted. … The Christian priesthood costs life, property, honor, friends and all worldly things. It cost Christ the same on the holy cross. ("First Sunday after Epiphany" from Complete Sermons of Martin Luther, vol. IV [Grand Rapids, MI:BakerBooks, 2007] p. 9)
In addition to this spiritual sacrifice—mortifying the deeds of the body—Peter mentions another, later on in the same chapter: "But ye are … a royal priesthood … that ye may show for the excellencies of him who called you out of darkness into his marvelous light." Here Peter touches upon the preaching office, the real sacrificial office, concerning which it is said (Ps 50, 23), "Whoso offereth the sacrifice of thanksgiving* glorifieth me." Preaching extols the grace of God. It is the offering of praise and thanks. Paul boasts (Rom 15, 16) that he sanctifies and offers the Gospel. (ibid., p. 11; ellipses are from original, as are Scripture references; * "Thanksgiving" here may be a reference to communion because "eucharist" is Greek for giving thanks.)