http://cambridgecitycoc.org.uk/2dk3.htm
This link is an article from David King 1819-1894, he has great insight
The Catholic priesthood isn't really supported in scripture. Yes there are mentions of priests in the New Testament, but there is nothing mentioning the Catholic version of priest. They see the priesthood as separated from the laity or common person.
We believe all Christians are part of the priesthood. Christ has made us a kingdom of priests.
In the Old Testament there was a priest separation, and this is where Catholics get the idea for their separated priesthood.
But Jews got their priesthood through sin, and Catholics don't realize this in copying them.
Jude may be mentioning their concept in a comment against separation of Christians.
Jud 1:19 TheseG3778 beG1526 they who separateG592 themselves,G1438 sensual,G5591 havingG2192 notG3361 the Spirit.G4151
"Separate" seems to allude to elevation above the common. So how do they get the theology of a separated priesthood? It comes from a promise in Exodud 19 that they tie to the old testament priesthood, but is likely a prophecy pointing to the Christian priesthood. The kingdom priesthood and peculiar people designation was not fulfilled in the Old Testament. It was conditional and they failed.
Exo 19:5 Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine:
Exo 19:6 And ye shall be unto me a kingdom of priests, and an holy nation. These are the words which thou shalt speak unto the children of Israel.
They were not obedient and did not keep the covenant, therefore the designation of a kingdom of priests was not achieved until Christ could cleanse the nation. Right after the promise in Exodus they built the golden calf and God relegated a partial priesthood. God in his foreknowledge instituted a temporary priesthood which we know as the Aaronic priesthood, this was a mercy to push sins forward instead of having instant justice. Sacrifices of the Aaronic priests were a remembrance of sins. Catholics simply pattern their priesthood incorrectly after the separated priesthood of Israel.
God caused the Aaronic separation because they were disobedient and could not be the priesthood in the promise, but there is no evidence God separated the Catholic priesthood. In the New Testament a separated priesthood was unnecessary.
Peter declared in confirming New Testament fulfillment,
1Pe 2:5 Ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ.
1Pe 2:9 But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light:
Then Peter declares that previously they were not the holy and peculiar people.
1Pe 2:10 Which in time past were not a people, but are now the people of God: which had not obtained mercy, but now have obtained mercy.
They were not a peculiar people and kingdom of priests in the Old Testament, so God appointed Aaron and his offspring as temporary priests. They could only become the true by accepting the prophet of Deuteronomy 18 as in Acts 3:20-24.
This temporary priesthood was insufficient and temporary. It could only push sins forward, not forgive them.
Heb 8:9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord.
They were never a kingdom of priests in God's eyes.
The New Testament priesthood isn't just a copy of the Old Testament.
Heb 7:12 For the priesthood being changed, there is made of necessity a change also of the law.
We are now a kingdom of priests.
Rev 1:6 And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen.
Revelation is addressed to all among them who are cleansed with Christ's blood. It verifies the priesthood of all believers cleansed by Christ. Just as in I Peter the context of a royal priesthood includes the entire holy nation.