It is always necessary to preface such an article with a reminder that God (and only God) forgives every sin. No matter how deliberate, malicious, destructive, insulting it is to God. As long as the sinner repents, humbles himself to God, and seeks sacramental absolution.
However, in the course of the Church’s life, there have been some “reserved sins” that cannot be immediately absolved by every confessor. Some require the consent of a prelate, and some require special faculties from the Pope himself.
Only God forgives sins but uses the Church as an instrument of reconciliation in the world. It is his will that the Church both preaches and administers his forgiveness to all men.
Only God forgives sins. Since he is the Son of God, Jesus says of himself, “The Son of man has authority on earth to forgive sins” and exercises this divine power: “Your sins are forgiven.” Further, by virtue of his divine authority he gives this power to men to exercise in his name.CCC 1441
The Church, who through the bishop and his priests forgives sins in the name of Jesus Christ and determines the manner of satisfaction, also prays for the sinner and does penance with him. Thus the sinner is healed and re-established in ecclesial communion.CCC 1448
This is a term used to describe those sins most confessors cant absolve. The church understands the sacrament as having a two-fold aspect to it, so these sins require extra acts of reconciliation with the Church. This is because those sins are very grave and cause harm to the Church, so to repair said harm, the penitent needs to seek reconciliation with his local bishop, or even the Vatican for graver offenses.
Certain particularly grave sins incur excommunication, the most severe ecclesiastical penalty, which impedes the reception of the sacraments and the exercise of certain ecclesiastical acts, and for which absolution consequently cannot be granted, according to canon law, except by the Pope, the bishop of the place or priests authorized by them. In danger of death any priest, even if deprived of faculties for hearing confessions, can absolve from every sin and excommunication.CCC 1463
Priests who sexually abuse minors:
A priest who in the act, on the occasion, or under the pretext of confession solicits a penitent to sin against the sixth commandment of the Decalogue is to be punished, according to the gravity of the delict, by suspension, prohibitions, and privations; in graver cases he is to be dismissed from the clerical state.Can. 1387
A cleric who in another way has committed an offense against the sixth commandment of the Decalogue, if the delict was committed by force or threats or publicly or with a minor below the age of sixteen years, is to be punished with just penalties, not excluding dismissal from the clerical state if the case so warrants.Can. 1395
Desecrating a Consecrated Host
A person who throws away the consecrated species or takes or retains them for a sacrilegious purpose incurs a latae sententiae excommunication reserved to the Apostolic See; moreover, a cleric can be punished with another penalty, not excluding dismissal from the clerical state.Can. 1367
Physically assaulting the Pope, a bishop or a priest
A person who uses physical force against the Roman Pontiff [as well as a bishop or priest] incurs a latae sententiae excommunication reserved to the Apostolic See.Can. 1370