In 2 Timothy 3:16-17, the Bible claims to be able to thoroughly equip us for every good work. For this reason, there should be nothing a Christian ought to do that is not mentioned, at least in passing, in one of the Biblical books. Yet not once are we told to confess our sins (mortal or venial, which are two categories of sin which aren't mentioned in the Bible either) to a priest. Rather, we are told to confess our sins to each other for purposes of prayer (James 5:16) and to Jesus Himself (1 John 1:9). It is nothing short of blasphemy to say a priest can forgive sin.
The Catholic priesthood actually mirrors the Pharisaical priesthood in several ways. The Catholic priests, for example, are addressed as "Father", just as the Pharisees were. Yet Jesus warns us against this title, which rightfully belongs to God alone (Matthew 23:9). In the New Testament, several roles are mentioned as being part of the Church structure. Elders, deacons, bishops and pastors. Yet there are no priests. Rather, we, as believers, are the priesthood (1 Peter 2:5-9), and we serve under one high priest, Christ Himself (Hebrews 4:14-15). Because of Him, we can boldly approach the throne of grace (Hebrews 4:16) because He is our mediator. Our only mediator (1 Timothy 2:5).
While I have specifically addressed Catholicism in this article, many religions preach that their "higher ups" are a necessary medium through which one must go to relate to God. But Christianity is unique in that it is a very personal religion. Because of the cross, every believer can have a personal relationship with God without ever having to go through a human or angelic mediator. The old order of priesthood is dead, and now, we are all priests.