First things first: Before you even consider reporting a priest for anything, you must be certain that the conduct in question is sufficiently grave and that you have credible evidence and firsthand testimony. Vague reports based on parish gossip are reprehensible. Keep in mind that it is no small matter to complain about a priest. As the priest acts in persona Christi (“in the person of Christ”) and is an alter Christus (“another Christ”), you should remember that someday Christ himself might ask of you, “Why did you persecute me?” (cf. Acts 9:4).
That said, if you do have credible evidence and firsthand testimony of sufficiently grave actions by a priest that must be reported, then who you report to depends on the status of the priest. If he is a diocesan priest, you would go to his bishop. If he belongs to a religious order, you would go to his religious superior.
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