Priestly faculties (aka canonical faculties) are permission to perform ministry and administer the sacraments. Sometimes faculties are only required to act licitly (e.g. the Eucharist) and other times they are required to act validly (e.g. Confession).
Ordination causes an ontological change in the man who receives it and thus can never be “undone.” A man ordained a priest is a priest forever. As Catholics we do not believe in the “lone ranger” form of ministry. All legitimate ministry is done in communion with the Bishops and Pope. If someone is lacking faculties that means he does not have permission to publicly minister or publicly administer the sacraments.
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