It is not easy to pinpoint for sure when the order of the priesthood got its start in the Catholic Church. One thing is for sure, though, it was never a part of the first church in the book of Acts or the first apostles, with all its pomp and priestly robes, sacrifice of the mass, celibacy, and confessional. Jesus did away with the order of the priesthood in the new covenant and gave His church the 5-fold ministry of apostles, prophets, evangelist, pastors, and teachers (Ephesians 4:11-12), and made every born-again believer a priest.
There was a sect called Nicolaitans that had sprung up in the first church. In Revelation 2:6, 15, Jesus rebuked them for their deeds and doctrines, which He said He hated. They believed that total separation between Christianity and paganism was not essential to be a Christian and that it was OK to participate in pagan festivals, eat food offered to idols and marry pagan spouses.
The word “Nicolaitan” means to “conquer the laity.” They had set up a priesthood-people caste system. It is believed that the forbidding of marriage of the priest had its start with their sect. This was a practice adapted from pagan priesthoods of that time. The scriptures call this practice a doctrine of devils (1 Timothy 4). It is believed that what began as a small group in the first century church mushroomed and became a strong dominant church by the 3rd and 4th century and became the foundation of the Roman Catholic church and the Papacy as we know it today.
It is also possible the priesthood had its start after the pagan emperor Constantine claimed himself to be head of the Christian church in Rome and gave himself the title of “Vicar of Christ.” He remained the head of the Roman pagan church, which also had its own pagan priesthood. So, it is easy to see how he brought it in to mimic the pagan priesthood system. I mean, why not; for he introduced so many other pagan practices into the church also, in what we know as the Roman Catholic church.
This elite society of a priesthood put themselves above everyone else and introduced the confessional box and their power to forgive sins (which only God the Father and Jesus the Son can do). They borrowed this practice from ancient paganism and not from the scriptures. This same confessional was a part of ancient Babylon paganism. It was part of their initiation into their religion, where one had to make confession to their pagan priesthood to be initiated into their ancient dark Mysteries (see The Two Babylons, by Alexander Hislop, pages 9-11).
“And have no fellowship with the unfruitful works of darkness, but rather reprove them”, Ephesians 5:11.
“But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanses us from all sin. …If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.”
1 John 1:1-9